third_party/v8/src/builtins/string-substr.tq

// Copyright 2019 the V8 project authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

namespace string {

// String.prototype.substr ( start, length )
// ES6 #sec-string.prototype.substr
transitioning javascript builtin StringPrototypeSubstr(
    js-implicit context: NativeContext, receiver: JSAny)(...arguments): String {
  const methodName: constexpr string = 'String.prototype.substr';
  // 1. Let O be ? RequireObjectCoercible(this value).
  // 2. Let S be ? ToString(O).
  const string: String = ToThisString(receiver, methodName);

  // 5. Let size be the number of code units in S.
  const size: uintptr = string.length_uintptr;

  // 3. Let intStart be ? ToInteger(start).
  // 6. If intStart < 0, set intStart to max(size + intStart, 0).
  const start = arguments[0];
  const initStart: uintptr =
      start != Undefined ? ConvertToRelativeIndex(start, size) : 0;

  // 4. If length is undefined,
  //   let end be +∞; otherwise let end be ? ToInteger(length).
  // 7. Let resultLength be min(max(end, 0), size - intStart).
  const length = arguments[1];
  const lengthLimit = size - initStart;
  assert(lengthLimit <= size);
  const resultLength: uintptr = length != Undefined ?
      ClampToIndexRange(length, lengthLimit) :
      lengthLimit;

  // 8. If resultLength ≤ 0, return the empty String "".
  if (resultLength == 0) return EmptyStringConstant();

  // 9. Return the String value containing resultLength consecutive code units
  // from S beginning with the code unit at index intStart.
  return SubString(string, initStart, initStart + resultLength);
}
}