src/third_party/devtools/node_modules/eslint/lib/rule-tester/rule-tester.js - cobalt - Git at Google

 /**
  * @fileoverview Mocha test wrapper
  * @author Ilya Volodin
  */
 "use strict";

 /* global describe, it */

 /*
  * This is a wrapper around mocha to allow for DRY unittests for eslint
  * Format:
  * RuleTester.run("{ruleName}", {
  *      valid: [
  *          "{code}",
  *          { code: "{code}", options: {options}, globals: {globals}, parser: "{parser}", settings: {settings} }
  *      ],
  *      invalid: [
  *          { code: "{code}", errors: {numErrors} },
  *          { code: "{code}", errors: ["{errorMessage}"] },
  *          { code: "{code}", options: {options}, globals: {globals}, parser: "{parser}", settings: {settings}, errors: [{ message: "{errorMessage}", type: "{errorNodeType}"}] }
  *      ]
  *  });
  *
  * Variables:
  * {code} - String that represents the code to be tested
  * {options} - Arguments that are passed to the configurable rules.
  * {globals} - An object representing a list of variables that are
  *             registered as globals
  * {parser} - String representing the parser to use
  * {settings} - An object representing global settings for all rules
  * {numErrors} - If failing case doesn't need to check error message,
  *               this integer will specify how many errors should be
  *               received
  * {errorMessage} - Message that is returned by the rule on failure
  * {errorNodeType} - AST node type that is returned by they rule as
  *                   a cause of the failure.
  */

 //------------------------------------------------------------------------------
 // Requirements
 //------------------------------------------------------------------------------

 const
     assert = require("assert"),
     path = require("path"),
     util = require("util"),
     lodash = require("lodash"),
     { getRuleOptionsSchema, validate } = require("../shared/config-validator"),
     { Linter, SourceCodeFixer, interpolate } = require("../linter");

 const ajv = require("../shared/ajv")({ strictDefaults: true });

 //------------------------------------------------------------------------------
 // Private Members
 //------------------------------------------------------------------------------

 /*
  * testerDefaultConfig must not be modified as it allows to reset the tester to
  * the initial default configuration
  */
 const testerDefaultConfig = { rules: {} };
 let defaultConfig = { rules: {} };

 /*
  * List every parameters possible on a test case that are not related to eslint
  * configuration
  */
 const RuleTesterParameters = [
     "code",
     "filename",
     "options",
     "errors",
     "output"
 ];

 const hasOwnProperty = Function.call.bind(Object.hasOwnProperty);

 /**
  * Clones a given value deeply.
  * Note: This ignores `parent` property.
  *
  * @param {any} x - A value to clone.
  * @returns {any} A cloned value.
  */
 function cloneDeeplyExcludesParent(x) {
     if (typeof x === "object" && x !== null) {
         if (Array.isArray(x)) {
             return x.map(cloneDeeplyExcludesParent);
         }

         const retv = {};

         for (const key in x) {
             if (key !== "parent" && hasOwnProperty(x, key)) {
                 retv[key] = cloneDeeplyExcludesParent(x[key]);
             }
         }

         return retv;
     }

     return x;
 }

 /**
  * Freezes a given value deeply.
  *
  * @param {any} x - A value to freeze.
  * @returns {void}
  */
 function freezeDeeply(x) {
     if (typeof x === "object" && x !== null) {
         if (Array.isArray(x)) {
             x.forEach(freezeDeeply);
         } else {
             for (const key in x) {
                 if (key !== "parent" && hasOwnProperty(x, key)) {
                     freezeDeeply(x[key]);
                 }
             }
         }
         Object.freeze(x);
     }
 }

 /**
  * Replace control characters by `\u00xx` form.
  * @param {string} text The text to sanitize.
  * @returns {string} The sanitized text.
  */
 function sanitize(text) {
     return text.replace(
         /[\u0000-\u001f]/gu, // eslint-disable-line no-control-regex
         c => `\\u${c.codePointAt(0).toString(16).padStart(4, "0")}`
     );
 }

 //------------------------------------------------------------------------------
 // Public Interface
 //------------------------------------------------------------------------------

 // default separators for testing
 const DESCRIBE = Symbol("describe");
 const IT = Symbol("it");

 /**
  * This is `it` default handler if `it` don't exist.
  * @this {Mocha}
  * @param {string} text - The description of the test case.
  * @param {Function} method - The logic of the test case.
  * @returns {any} Returned value of `method`.
  */
 function itDefaultHandler(text, method) {
     try {
         return method.call(this);
     } catch (err) {
         if (err instanceof assert.AssertionError) {
             err.message += ` (${util.inspect(err.actual)} ${err.operator} ${util.inspect(err.expected)})`;
         }
         throw err;
     }
 }

 /**
  * This is `describe` default handler if `describe` don't exist.
  * @this {Mocha}
  * @param {string} text - The description of the test case.
  * @param {Function} method - The logic of the test case.
  * @returns {any} Returned value of `method`.
  */
 function describeDefaultHandler(text, method) {
     return method.call(this);
 }

 class RuleTester {

     /**
      * Creates a new instance of RuleTester.
      * @param {Object} [testerConfig] Optional, extra configuration for the tester
      * @constructor
      */
     constructor(testerConfig) {

         /**
          * The configuration to use for this tester. Combination of the tester
          * configuration and the default configuration.
          * @type {Object}
          */
         this.testerConfig = lodash.merge(

             // we have to clone because merge uses the first argument for recipient
             lodash.cloneDeep(defaultConfig),
             testerConfig,
             { rules: { "rule-tester/validate-ast": "error" } }
         );

         /**
          * Rule definitions to define before tests.
          * @type {Object}
          */
         this.rules = {};
         this.linter = new Linter();
     }

     /**
      * Set the configuration to use for all future tests
      * @param {Object} config the configuration to use.
      * @returns {void}
      */
     static setDefaultConfig(config) {
         if (typeof config !== "object") {
             throw new TypeError("RuleTester.setDefaultConfig: config must be an object");
         }
         defaultConfig = config;

         // Make sure the rules object exists since it is assumed to exist later
         defaultConfig.rules = defaultConfig.rules || {};
     }

     /**
      * Get the current configuration used for all tests
      * @returns {Object} the current configuration
      */
     static getDefaultConfig() {
         return defaultConfig;
     }

     /**
      * Reset the configuration to the initial configuration of the tester removing
      * any changes made until now.
      * @returns {void}
      */
     static resetDefaultConfig() {
         defaultConfig = lodash.cloneDeep(testerDefaultConfig);
     }


     /*
      * If people use `mocha test.js --watch` command, `describe` and `it` function
      * instances are different for each execution. So `describe` and `it` should get fresh instance
      * always.
      */
     static get describe() {
         return (
             this[DESCRIBE] ||
             (typeof describe === "function" ? describe : describeDefaultHandler)
         );
     }

     static set describe(value) {
         this[DESCRIBE] = value;
     }

     static get it() {
         return (
             this[IT] ||
             (typeof it === "function" ? it : itDefaultHandler)
         );
     }

     static set it(value) {
         this[IT] = value;
     }

     /**
      * Define a rule for one particular run of tests.
      * @param {string} name The name of the rule to define.
      * @param {Function} rule The rule definition.
      * @returns {void}
      */
     defineRule(name, rule) {
         this.rules[name] = rule;
     }

     /**
      * Adds a new rule test to execute.
      * @param {string} ruleName The name of the rule to run.
      * @param {Function} rule The rule to test.
      * @param {Object} test The collection of tests to run.
      * @returns {void}
      */
     run(ruleName, rule, test) {

         const testerConfig = this.testerConfig,
             requiredScenarios = ["valid", "invalid"],
             scenarioErrors = [],
             linter = this.linter;

         if (lodash.isNil(test) || typeof test !== "object") {
             throw new TypeError(`Test Scenarios for rule ${ruleName} : Could not find test scenario object`);
         }

         requiredScenarios.forEach(scenarioType => {
             if (lodash.isNil(test[scenarioType])) {
                 scenarioErrors.push(`Could not find any ${scenarioType} test scenarios`);
             }
         });

         if (scenarioErrors.length > 0) {
             throw new Error([
                 `Test Scenarios for rule ${ruleName} is invalid:`
             ].concat(scenarioErrors).join("\n"));
         }


         linter.defineRule(ruleName, Object.assign({}, rule, {

             // Create a wrapper rule that freezes the `context` properties.
             create(context) {
                 freezeDeeply(context.options);
                 freezeDeeply(context.settings);
                 freezeDeeply(context.parserOptions);

                 return (typeof rule === "function" ? rule : rule.create)(context);
             }
         }));

         linter.defineRules(this.rules);

         /**
          * Run the rule for the given item
          * @param {string|Object} item Item to run the rule against
          * @returns {Object} Eslint run result
          * @private
          */
         function runRuleForItem(item) {
             let config = lodash.cloneDeep(testerConfig),
                 code, filename, output, beforeAST, afterAST;

             if (typeof item === "string") {
                 code = item;
             } else {
                 code = item.code;

                 /*
                  * Assumes everything on the item is a config except for the
                  * parameters used by this tester
                  */
                 const itemConfig = lodash.omit(item, RuleTesterParameters);

                 /*
                  * Create the config object from the tester config and this item
                  * specific configurations.
                  */
                 config = lodash.merge(
                     config,
                     itemConfig
                 );
             }

             if (item.filename) {
                 filename = item.filename;
             }

             if (Object.prototype.hasOwnProperty.call(item, "options")) {
                 assert(Array.isArray(item.options), "options must be an array");
                 config.rules[ruleName] = [1].concat(item.options);
             } else {
                 config.rules[ruleName] = 1;
             }

             const schema = getRuleOptionsSchema(rule);

             /*
              * Setup AST getters.
              * The goal is to check whether or not AST was modified when
              * running the rule under test.
              */
             linter.defineRule("rule-tester/validate-ast", () => ({
                 Program(node) {
                     beforeAST = cloneDeeplyExcludesParent(node);
                 },
                 "Program:exit"(node) {
                     afterAST = node;
                 }
             }));

             if (typeof config.parser === "string") {
                 assert(path.isAbsolute(config.parser), "Parsers provided as strings to RuleTester must be absolute paths");
                 linter.defineParser(config.parser, require(config.parser));
             }

             if (schema) {
                 ajv.validateSchema(schema);

                 if (ajv.errors) {
                     const errors = ajv.errors.map(error => {
                         const field = error.dataPath[0] === "." ? error.dataPath.slice(1) : error.dataPath;

                         return `\t${field}: ${error.message}`;
                     }).join("\n");

                     throw new Error([`Schema for rule ${ruleName} is invalid:`, errors]);
                 }

                 /*
                  * `ajv.validateSchema` checks for errors in the structure of the schema (by comparing the schema against a "meta-schema"),
                  * and it reports those errors individually. However, there are other types of schema errors that only occur when compiling
                  * the schema (e.g. using invalid defaults in a schema), and only one of these errors can be reported at a time. As a result,
                  * the schema is compiled here separately from checking for `validateSchema` errors.
                  */
                 try {
                     ajv.compile(schema);
                 } catch (err) {
                     throw new Error(`Schema for rule ${ruleName} is invalid: ${err.message}`);
                 }
             }

             validate(config, "rule-tester", id => (id === ruleName ? rule : null));

             // Verify the code.
             const messages = linter.verify(code, config, filename);

             // Ignore syntax errors for backward compatibility if `errors` is a number.
             if (typeof item.errors !== "number") {
                 const errorMessage = messages.find(m => m.fatal);

                 assert(!errorMessage, `A fatal parsing error occurred: ${errorMessage && errorMessage.message}`);
             }

             // Verify if autofix makes a syntax error or not.
             if (messages.some(m => m.fix)) {
                 output = SourceCodeFixer.applyFixes(code, messages).output;
                 const errorMessageInFix = linter.verify(output, config, filename).find(m => m.fatal);

                 assert(!errorMessageInFix, `A fatal parsing error occurred in autofix: ${errorMessageInFix && errorMessageInFix.message}`);
             } else {
                 output = code;
             }

             return {
                 messages,
                 output,
                 beforeAST,
                 afterAST: cloneDeeplyExcludesParent(afterAST)
             };
         }

         /**
          * Check if the AST was changed
          * @param {ASTNode} beforeAST AST node before running
          * @param {ASTNode} afterAST AST node after running
          * @returns {void}
          * @private
          */
         function assertASTDidntChange(beforeAST, afterAST) {
             if (!lodash.isEqual(beforeAST, afterAST)) {
                 assert.fail("Rule should not modify AST.");
             }
         }

         /**
          * Check if the template is valid or not
          * all valid cases go through this
          * @param {string|Object} item Item to run the rule against
          * @returns {void}
          * @private
          */
         function testValidTemplate(item) {
             const result = runRuleForItem(item);
             const messages = result.messages;

             assert.strictEqual(messages.length, 0, util.format("Should have no errors but had %d: %s",
                 messages.length, util.inspect(messages)));

             assertASTDidntChange(result.beforeAST, result.afterAST);
         }

         /**
          * Asserts that the message matches its expected value. If the expected
          * value is a regular expression, it is checked against the actual
          * value.
          * @param {string} actual Actual value
          * @param {string|RegExp} expected Expected value
          * @returns {void}
          * @private
          */
         function assertMessageMatches(actual, expected) {
             if (expected instanceof RegExp) {

                 // assert.js doesn't have a built-in RegExp match function
                 assert.ok(
                     expected.test(actual),
                     `Expected '${actual}' to match ${expected}`
                 );
             } else {
                 assert.strictEqual(actual, expected);
             }
         }

         /**
          * Check if the template is invalid or not
          * all invalid cases go through this.
          * @param {string|Object} item Item to run the rule against
          * @returns {void}
          * @private
          */
         function testInvalidTemplate(item) {
             assert.ok(item.errors || item.errors === 0,
                 `Did not specify errors for an invalid test of ${ruleName}`);

             const result = runRuleForItem(item);
             const messages = result.messages;


             if (typeof item.errors === "number") {
                 assert.strictEqual(messages.length, item.errors, util.format("Should have %d error%s but had %d: %s",
                     item.errors, item.errors === 1 ? "" : "s", messages.length, util.inspect(messages)));
             } else {
                 assert.strictEqual(
                     messages.length, item.errors.length,
                     util.format(
                         "Should have %d error%s but had %d: %s",
                         item.errors.length, item.errors.length === 1 ? "" : "s", messages.length, util.inspect(messages)
                     )
                 );

                 const hasMessageOfThisRule = messages.some(m => m.ruleId === ruleName);

                 for (let i = 0, l = item.errors.length; i < l; i++) {
                     const error = item.errors[i];
                     const message = messages[i];

                     assert(hasMessageOfThisRule, "Error rule name should be the same as the name of the rule being tested");

                     if (typeof error === "string" || error instanceof RegExp) {

                         // Just an error message.
                         assertMessageMatches(message.message, error);
                     } else if (typeof error === "object") {

                         /*
                          * Error object.
                          * This may have a message, messageId, data, node type, line, and/or
                          * column.
                          */
                         if (hasOwnProperty(error, "message")) {
                             assert.ok(!hasOwnProperty(error, "messageId"), "Error should not specify both 'message' and a 'messageId'.");
                             assert.ok(!hasOwnProperty(error, "data"), "Error should not specify both 'data' and 'message'.");
                             assertMessageMatches(message.message, error.message);
                         } else if (hasOwnProperty(error, "messageId")) {
                             assert.ok(
                                 hasOwnProperty(rule, "meta") && hasOwnProperty(rule.meta, "messages"),
                                 "Error can not use 'messageId' if rule under test doesn't define 'meta.messages'."
                             );
                             if (!hasOwnProperty(rule.meta.messages, error.messageId)) {
                                 const friendlyIDList = `[${Object.keys(rule.meta.messages).map(key => `'${key}'`).join(", ")}]`;

                                 assert(false, `Invalid messageId '${error.messageId}'. Expected one of ${friendlyIDList}.`);
                             }
                             assert.strictEqual(
                                 error.messageId,
                                 message.messageId,
                                 `messageId '${message.messageId}' does not match expected messageId '${error.messageId}'.`
                             );
                             if (hasOwnProperty(error, "data")) {

                                 /*
                                  *  if data was provided, then directly compare the returned message to a synthetic
                                  *  interpolated message using the same message ID and data provided in the test.
                                  *  See https://github.com/eslint/eslint/issues/9890 for context.
                                  */
                                 const unformattedOriginalMessage = rule.meta.messages[error.messageId];
                                 const rehydratedMessage = interpolate(unformattedOriginalMessage, error.data);

                                 assert.strictEqual(
                                     message.message,
                                     rehydratedMessage,
                                     `Hydrated message "${rehydratedMessage}" does not match "${message.message}"`
                                 );
                             }
                         }

                         assert.ok(
                             hasOwnProperty(error, "data") ? hasOwnProperty(error, "messageId") : true,
                             "Error must specify 'messageId' if 'data' is used."
                         );

                         if (error.type) {
                             assert.strictEqual(message.nodeType, error.type, `Error type should be ${error.type}, found ${message.nodeType}`);
                         }

                         if (Object.prototype.hasOwnProperty.call(error, "line")) {
                             assert.strictEqual(message.line, error.line, `Error line should be ${error.line}`);
                         }

                         if (Object.prototype.hasOwnProperty.call(error, "column")) {
                             assert.strictEqual(message.column, error.column, `Error column should be ${error.column}`);
                         }

                         if (Object.prototype.hasOwnProperty.call(error, "endLine")) {
                             assert.strictEqual(message.endLine, error.endLine, `Error endLine should be ${error.endLine}`);
                         }

                         if (Object.prototype.hasOwnProperty.call(error, "endColumn")) {
                             assert.strictEqual(message.endColumn, error.endColumn, `Error endColumn should be ${error.endColumn}`);
                         }
                     } else {

                         // Message was an unexpected type
                         assert.fail(`Error should be a string, object, or RegExp, but found (${util.inspect(message)})`);
                     }
                 }
             }

             if (Object.prototype.hasOwnProperty.call(item, "output")) {
                 if (item.output === null) {
                     assert.strictEqual(
                         result.output,
                         item.code,
                         "Expected no autofixes to be suggested"
                     );
                 } else {
                     assert.strictEqual(result.output, item.output, "Output is incorrect.");
                 }
             }

             assertASTDidntChange(result.beforeAST, result.afterAST);
         }

         /*
          * This creates a mocha test suite and pipes all supplied info through
          * one of the templates above.
          */
         RuleTester.describe(ruleName, () => {
             RuleTester.describe("valid", () => {
                 test.valid.forEach(valid => {
                     RuleTester.it(sanitize(typeof valid === "object" ? valid.code : valid), () => {
                         testValidTemplate(valid);
                     });
                 });
             });

             RuleTester.describe("invalid", () => {
                 test.invalid.forEach(invalid => {
                     RuleTester.it(sanitize(invalid.code), () => {
                         testInvalidTemplate(invalid);
                     });
                 });
             });
         });
     }
 }

 RuleTester[DESCRIBE] = RuleTester[IT] = null;

 module.exports = RuleTester;
	/**
	* @fileoverview Mocha test wrapper
	* @author Ilya Volodin
	*/
	"use strict";

	/* global describe, it */

	/*
	* This is a wrapper around mocha to allow for DRY unittests for eslint
	* Format:
	* RuleTester.run("{ruleName}", {
	* valid: [
	* "{code}",
	* { code: "{code}", options: {options}, globals: {globals}, parser: "{parser}", settings: {settings} }
	* ],
	* invalid: [
	* { code: "{code}", errors: {numErrors} },
	* { code: "{code}", errors: ["{errorMessage}"] },
	* { code: "{code}", options: {options}, globals: {globals}, parser: "{parser}", settings: {settings}, errors: [{ message: "{errorMessage}", type: "{errorNodeType}"}] }
	* ]
	* });
	*
	* Variables:
	* {code} - String that represents the code to be tested
	* {options} - Arguments that are passed to the configurable rules.
	* {globals} - An object representing a list of variables that are
	* registered as globals
	* {parser} - String representing the parser to use
	* {settings} - An object representing global settings for all rules
	* {numErrors} - If failing case doesn't need to check error message,
	* this integer will specify how many errors should be
	* received
	* {errorMessage} - Message that is returned by the rule on failure
	* {errorNodeType} - AST node type that is returned by they rule as
	* a cause of the failure.
	*/

	//------------------------------------------------------------------------------
	// Requirements
	//------------------------------------------------------------------------------

	const
	assert = require("assert"),
	path = require("path"),
	util = require("util"),
	lodash = require("lodash"),
	{ getRuleOptionsSchema, validate } = require("../shared/config-validator"),
	{ Linter, SourceCodeFixer, interpolate } = require("../linter");

	const ajv = require("../shared/ajv")({ strictDefaults: true });

	//------------------------------------------------------------------------------
	// Private Members
	//------------------------------------------------------------------------------

	/*
	* testerDefaultConfig must not be modified as it allows to reset the tester to
	* the initial default configuration
	*/
	const testerDefaultConfig = { rules: {} };
	let defaultConfig = { rules: {} };

	/*
	* List every parameters possible on a test case that are not related to eslint
	* configuration
	*/
	const RuleTesterParameters = [
	"code",
	"filename",
	"options",
	"errors",
	"output"
	];

	const hasOwnProperty = Function.call.bind(Object.hasOwnProperty);

	/**
	* Clones a given value deeply.
	* Note: This ignores `parent` property.
	*
	* @param {any} x - A value to clone.
	* @returns {any} A cloned value.
	*/
	function cloneDeeplyExcludesParent(x) {
	if (typeof x === "object" && x !== null) {
	if (Array.isArray(x)) {
	return x.map(cloneDeeplyExcludesParent);
	}

	const retv = {};

	for (const key in x) {
	if (key !== "parent" && hasOwnProperty(x, key)) {
	retv[key] = cloneDeeplyExcludesParent(x[key]);
	}
	}

	return retv;
	}

	return x;
	}

	/**
	* Freezes a given value deeply.
	*
	* @param {any} x - A value to freeze.
	* @returns {void}
	*/
	function freezeDeeply(x) {
	if (typeof x === "object" && x !== null) {
	if (Array.isArray(x)) {
	x.forEach(freezeDeeply);
	} else {
	for (const key in x) {
	if (key !== "parent" && hasOwnProperty(x, key)) {
	freezeDeeply(x[key]);
	}
	}
	}
	Object.freeze(x);
	}
	}

	/**
	* Replace control characters by `\u00xx` form.
	* @param {string} text The text to sanitize.
	* @returns {string} The sanitized text.
	*/
	function sanitize(text) {
	return text.replace(
	/[\u0000-\u001f]/gu, // eslint-disable-line no-control-regex
	c => `\\u${c.codePointAt(0).toString(16).padStart(4, "0")}`
	);
	}

	//------------------------------------------------------------------------------
	// Public Interface
	//------------------------------------------------------------------------------

	// default separators for testing
	const DESCRIBE = Symbol("describe");
	const IT = Symbol("it");

	/**
	* This is `it` default handler if `it` don't exist.
	* @this {Mocha}
	* @param {string} text - The description of the test case.
	* @param {Function} method - The logic of the test case.
	* @returns {any} Returned value of `method`.
	*/
	function itDefaultHandler(text, method) {
	try {
	return method.call(this);
	} catch (err) {
	if (err instanceof assert.AssertionError) {
	err.message += ` (${util.inspect(err.actual)} ${err.operator} ${util.inspect(err.expected)})`;
	}
	throw err;
	}
	}

	/**
	* This is `describe` default handler if `describe` don't exist.
	* @this {Mocha}
	* @param {string} text - The description of the test case.
	* @param {Function} method - The logic of the test case.
	* @returns {any} Returned value of `method`.
	*/
	function describeDefaultHandler(text, method) {
	return method.call(this);
	}

	class RuleTester {

	/**
	* Creates a new instance of RuleTester.
	* @param {Object} [testerConfig] Optional, extra configuration for the tester
	* @constructor
	*/
	constructor(testerConfig) {

	/**
	* The configuration to use for this tester. Combination of the tester
	* configuration and the default configuration.
	* @type {Object}
	*/
	this.testerConfig = lodash.merge(

	// we have to clone because merge uses the first argument for recipient
	lodash.cloneDeep(defaultConfig),
	testerConfig,
	{ rules: { "rule-tester/validate-ast": "error" } }
	);

	/**
	* Rule definitions to define before tests.
	* @type {Object}
	*/
	this.rules = {};
	this.linter = new Linter();
	}

	/**
	* Set the configuration to use for all future tests
	* @param {Object} config the configuration to use.
	* @returns {void}
	*/
	static setDefaultConfig(config) {
	if (typeof config !== "object") {
	throw new TypeError("RuleTester.setDefaultConfig: config must be an object");
	}
	defaultConfig = config;

	// Make sure the rules object exists since it is assumed to exist later
	defaultConfig.rules = defaultConfig.rules \|\| {};
	}

	/**
	* Get the current configuration used for all tests
	* @returns {Object} the current configuration
	*/
	static getDefaultConfig() {
	return defaultConfig;
	}

	/**
	* Reset the configuration to the initial configuration of the tester removing
	* any changes made until now.
	* @returns {void}
	*/
	static resetDefaultConfig() {
	defaultConfig = lodash.cloneDeep(testerDefaultConfig);
	}


	/*
	* If people use `mocha test.js --watch` command, `describe` and `it` function
	* instances are different for each execution. So `describe` and `it` should get fresh instance
	* always.
	*/
	static get describe() {
	return (
	this[DESCRIBE] \|\|
	(typeof describe === "function" ? describe : describeDefaultHandler)
	);
	}

	static set describe(value) {
	this[DESCRIBE] = value;
	}

	static get it() {
	return (
	this[IT] \|\|
	(typeof it === "function" ? it : itDefaultHandler)
	);
	}

	static set it(value) {
	this[IT] = value;
	}

	/**
	* Define a rule for one particular run of tests.
	* @param {string} name The name of the rule to define.
	* @param {Function} rule The rule definition.
	* @returns {void}
	*/
	defineRule(name, rule) {
	this.rules[name] = rule;
	}

	/**
	* Adds a new rule test to execute.
	* @param {string} ruleName The name of the rule to run.
	* @param {Function} rule The rule to test.
	* @param {Object} test The collection of tests to run.
	* @returns {void}
	*/
	run(ruleName, rule, test) {

	const testerConfig = this.testerConfig,
	requiredScenarios = ["valid", "invalid"],
	scenarioErrors = [],
	linter = this.linter;

	if (lodash.isNil(test) \|\| typeof test !== "object") {
	throw new TypeError(`Test Scenarios for rule ${ruleName} : Could not find test scenario object`);
	}

	requiredScenarios.forEach(scenarioType => {
	if (lodash.isNil(test[scenarioType])) {
	scenarioErrors.push(`Could not find any ${scenarioType} test scenarios`);
	}
	});

	if (scenarioErrors.length > 0) {
	throw new Error([
	`Test Scenarios for rule ${ruleName} is invalid:`
	].concat(scenarioErrors).join("\n"));
	}


	linter.defineRule(ruleName, Object.assign({}, rule, {

	// Create a wrapper rule that freezes the `context` properties.
	create(context) {
	freezeDeeply(context.options);
	freezeDeeply(context.settings);
	freezeDeeply(context.parserOptions);

	return (typeof rule === "function" ? rule : rule.create)(context);
	}
	}));

	linter.defineRules(this.rules);

	/**
	* Run the rule for the given item
	* @param {string\|Object} item Item to run the rule against
	* @returns {Object} Eslint run result
	* @private
	*/
	function runRuleForItem(item) {
	let config = lodash.cloneDeep(testerConfig),
	code, filename, output, beforeAST, afterAST;

	if (typeof item === "string") {
	code = item;
	} else {
	code = item.code;

	/*
	* Assumes everything on the item is a config except for the
	* parameters used by this tester
	*/
	const itemConfig = lodash.omit(item, RuleTesterParameters);

	/*
	* Create the config object from the tester config and this item
	* specific configurations.
	*/
	config = lodash.merge(
	config,
	itemConfig
	);
	}

	if (item.filename) {
	filename = item.filename;
	}

	if (Object.prototype.hasOwnProperty.call(item, "options")) {
	assert(Array.isArray(item.options), "options must be an array");
	config.rules[ruleName] = [1].concat(item.options);
	} else {
	config.rules[ruleName] = 1;
	}

	const schema = getRuleOptionsSchema(rule);

	/*
	* Setup AST getters.
	* The goal is to check whether or not AST was modified when
	* running the rule under test.
	*/
	linter.defineRule("rule-tester/validate-ast", () => ({
	Program(node) {
	beforeAST = cloneDeeplyExcludesParent(node);
	},
	"Program:exit"(node) {
	afterAST = node;
	}
	}));

	if (typeof config.parser === "string") {
	assert(path.isAbsolute(config.parser), "Parsers provided as strings to RuleTester must be absolute paths");
	linter.defineParser(config.parser, require(config.parser));
	}

	if (schema) {
	ajv.validateSchema(schema);

	if (ajv.errors) {
	const errors = ajv.errors.map(error => {
	const field = error.dataPath[0] === "." ? error.dataPath.slice(1) : error.dataPath;

	return `\t${field}: ${error.message}`;
	}).join("\n");

	throw new Error([`Schema for rule ${ruleName} is invalid:`, errors]);
	}

	/*
	* `ajv.validateSchema` checks for errors in the structure of the schema (by comparing the schema against a "meta-schema"),
	* and it reports those errors individually. However, there are other types of schema errors that only occur when compiling
	* the schema (e.g. using invalid defaults in a schema), and only one of these errors can be reported at a time. As a result,
	* the schema is compiled here separately from checking for `validateSchema` errors.
	*/
	try {
	ajv.compile(schema);
	} catch (err) {
	throw new Error(`Schema for rule ${ruleName} is invalid: ${err.message}`);
	}
	}

	validate(config, "rule-tester", id => (id === ruleName ? rule : null));

	// Verify the code.
	const messages = linter.verify(code, config, filename);

	// Ignore syntax errors for backward compatibility if `errors` is a number.
	if (typeof item.errors !== "number") {
	const errorMessage = messages.find(m => m.fatal);

	assert(!errorMessage, `A fatal parsing error occurred: ${errorMessage && errorMessage.message}`);
	}

	// Verify if autofix makes a syntax error or not.
	if (messages.some(m => m.fix)) {
	output = SourceCodeFixer.applyFixes(code, messages).output;
	const errorMessageInFix = linter.verify(output, config, filename).find(m => m.fatal);

	assert(!errorMessageInFix, `A fatal parsing error occurred in autofix: ${errorMessageInFix && errorMessageInFix.message}`);
	} else {
	output = code;
	}

	return {
	messages,
	output,
	beforeAST,
	afterAST: cloneDeeplyExcludesParent(afterAST)
	};
	}

	/**
	* Check if the AST was changed
	* @param {ASTNode} beforeAST AST node before running
	* @param {ASTNode} afterAST AST node after running
	* @returns {void}
	* @private
	*/
	function assertASTDidntChange(beforeAST, afterAST) {
	if (!lodash.isEqual(beforeAST, afterAST)) {
	assert.fail("Rule should not modify AST.");
	}
	}

	/**
	* Check if the template is valid or not
	* all valid cases go through this
	* @param {string\|Object} item Item to run the rule against
	* @returns {void}
	* @private
	*/
	function testValidTemplate(item) {
	const result = runRuleForItem(item);
	const messages = result.messages;

	assert.strictEqual(messages.length, 0, util.format("Should have no errors but had %d: %s",
	messages.length, util.inspect(messages)));

	assertASTDidntChange(result.beforeAST, result.afterAST);
	}

	/**
	* Asserts that the message matches its expected value. If the expected
	* value is a regular expression, it is checked against the actual
	* value.
	* @param {string} actual Actual value
	* @param {string\|RegExp} expected Expected value
	* @returns {void}
	* @private
	*/
	function assertMessageMatches(actual, expected) {
	if (expected instanceof RegExp) {

	// assert.js doesn't have a built-in RegExp match function
	assert.ok(
	expected.test(actual),
	`Expected '${actual}' to match ${expected}`
	);
	} else {
	assert.strictEqual(actual, expected);
	}
	}

	/**
	* Check if the template is invalid or not
	* all invalid cases go through this.
	* @param {string\|Object} item Item to run the rule against
	* @returns {void}
	* @private
	*/
	function testInvalidTemplate(item) {
	assert.ok(item.errors \|\| item.errors === 0,
	`Did not specify errors for an invalid test of ${ruleName}`);

	const result = runRuleForItem(item);
	const messages = result.messages;


	if (typeof item.errors === "number") {
	assert.strictEqual(messages.length, item.errors, util.format("Should have %d error%s but had %d: %s",
	item.errors, item.errors === 1 ? "" : "s", messages.length, util.inspect(messages)));
	} else {
	assert.strictEqual(
	messages.length, item.errors.length,
	util.format(
	"Should have %d error%s but had %d: %s",
	item.errors.length, item.errors.length === 1 ? "" : "s", messages.length, util.inspect(messages)
	)
	);

	const hasMessageOfThisRule = messages.some(m => m.ruleId === ruleName);

	for (let i = 0, l = item.errors.length; i < l; i++) {
	const error = item.errors[i];
	const message = messages[i];

	assert(hasMessageOfThisRule, "Error rule name should be the same as the name of the rule being tested");

	if (typeof error === "string" \|\| error instanceof RegExp) {

	// Just an error message.
	assertMessageMatches(message.message, error);
	} else if (typeof error === "object") {

	/*
	* Error object.
	* This may have a message, messageId, data, node type, line, and/or
	* column.
	*/
	if (hasOwnProperty(error, "message")) {
	assert.ok(!hasOwnProperty(error, "messageId"), "Error should not specify both 'message' and a 'messageId'.");
	assert.ok(!hasOwnProperty(error, "data"), "Error should not specify both 'data' and 'message'.");
	assertMessageMatches(message.message, error.message);
	} else if (hasOwnProperty(error, "messageId")) {
	assert.ok(
	hasOwnProperty(rule, "meta") && hasOwnProperty(rule.meta, "messages"),
	"Error can not use 'messageId' if rule under test doesn't define 'meta.messages'."
	);
	if (!hasOwnProperty(rule.meta.messages, error.messageId)) {
	const friendlyIDList = `[${Object.keys(rule.meta.messages).map(key => `'${key}'`).join(", ")}]`;

	assert(false, `Invalid messageId '${error.messageId}'. Expected one of ${friendlyIDList}.`);
	}
	assert.strictEqual(
	error.messageId,
	message.messageId,
	`messageId '${message.messageId}' does not match expected messageId '${error.messageId}'.`
	);
	if (hasOwnProperty(error, "data")) {

	/*
	* if data was provided, then directly compare the returned message to a synthetic
	* interpolated message using the same message ID and data provided in the test.
	* See https://github.com/eslint/eslint/issues/9890 for context.
	*/
	const unformattedOriginalMessage = rule.meta.messages[error.messageId];
	const rehydratedMessage = interpolate(unformattedOriginalMessage, error.data);

	assert.strictEqual(
	message.message,
	rehydratedMessage,
	`Hydrated message "${rehydratedMessage}" does not match "${message.message}"`
	);
	}
	}

	assert.ok(
	hasOwnProperty(error, "data") ? hasOwnProperty(error, "messageId") : true,
	"Error must specify 'messageId' if 'data' is used."
	);

	if (error.type) {
	assert.strictEqual(message.nodeType, error.type, `Error type should be ${error.type}, found ${message.nodeType}`);
	}

	if (Object.prototype.hasOwnProperty.call(error, "line")) {
	assert.strictEqual(message.line, error.line, `Error line should be ${error.line}`);
	}

	if (Object.prototype.hasOwnProperty.call(error, "column")) {
	assert.strictEqual(message.column, error.column, `Error column should be ${error.column}`);
	}

	if (Object.prototype.hasOwnProperty.call(error, "endLine")) {
	assert.strictEqual(message.endLine, error.endLine, `Error endLine should be ${error.endLine}`);
	}

	if (Object.prototype.hasOwnProperty.call(error, "endColumn")) {
	assert.strictEqual(message.endColumn, error.endColumn, `Error endColumn should be ${error.endColumn}`);
	}
	} else {

	// Message was an unexpected type
	assert.fail(`Error should be a string, object, or RegExp, but found (${util.inspect(message)})`);
	}
	}
	}

	if (Object.prototype.hasOwnProperty.call(item, "output")) {
	if (item.output === null) {
	assert.strictEqual(
	result.output,
	item.code,
	"Expected no autofixes to be suggested"
	);
	} else {
	assert.strictEqual(result.output, item.output, "Output is incorrect.");
	}
	}

	assertASTDidntChange(result.beforeAST, result.afterAST);
	}

	/*
	* This creates a mocha test suite and pipes all supplied info through
	* one of the templates above.
	*/
	RuleTester.describe(ruleName, () => {
	RuleTester.describe("valid", () => {
	test.valid.forEach(valid => {
	RuleTester.it(sanitize(typeof valid === "object" ? valid.code : valid), () => {
	testValidTemplate(valid);
	});
	});
	});

	RuleTester.describe("invalid", () => {
	test.invalid.forEach(invalid => {
	RuleTester.it(sanitize(invalid.code), () => {
	testInvalidTemplate(invalid);
	});
	});
	});
	});
	}
	}

	RuleTester[DESCRIBE] = RuleTester[IT] = null;

	module.exports = RuleTester;