src/third_party/mozjs-45/js/src/jit-test/tests/arrow-functions/params-2.js

// (a) => expr

var f = (a) => 2 * a;  // parens are allowed
assertEq(f(12), 24);
var g = (a, b) => a + b;
assertEq([1, 2, 3, 4, 5].reduce((a, b) => a + b), 15);