[New] Add Array#indexOf from post-ES6 errata.

See tc39/ecma262#316.

README.md

@@ -70,6 +70,7 @@ In both browser and node you may also want to include `unorm`; see the [`String.
     * `findIndex()` ([a standalone shim is also available](https://github.com/paulmillr/Array.prototype.findIndex))
     * `keys()` (note: keys/values/entries return an `ArrayIterator` object)
     * `values()`
+    * `indexOf()` (ES6 errata)
 * `Object`:
     * `assign()` ([a standalone shim is also available](https://github.com/ljharb/object.assign))
     * `is()` ([a standalone shim is also available](https://github.com/ljharb/object-is))

es6-shim.js

@@ -183,6 +183,7 @@
   var globals = getGlobal();
   var globalIsFinite = globals.isFinite;
   var _indexOf = Function.call.bind(String.prototype.indexOf);
+  var _arrayIndexOfApply = Function.apply.bind(Array.prototype.indexOf);
   var _concat = Function.call.bind(Array.prototype.concat);
   var _sort = Function.call.bind(Array.prototype.sort);
   var _strSlice = Function.call.bind(String.prototype.slice);
@@ -1200,6 +1201,18 @@
   }
   defineProperties(Array.prototype, ArrayPrototypeShims);
 
+  if (1 / [true].indexOf(true, -0) < 0) {
+    // indexOf when given a position arg of -0 should return +0.
+    // https://github.com/tc39/ecma262/pull/316
+    defineProperty(Array.prototype, 'indexOf', function indexOf(searchElement) {
+      var value = _arrayIndexOfApply(this, arguments);
+      if (value === 0 && (1 / value) < 0) {
+        return 0;
+      }
+      return value;
+    }, true);
+  }
+
   addIterator(Array.prototype, function () { return this.values(); });
   // Chrome defines keys/values/entries on Array, but doesn't give us
   // any way to identify its iterator.  So add our own shimmed field.

test/array.js

@@ -14,6 +14,10 @@ var runArrayTests = function (it) {
   var ifSymbolUnscopablesIt = isSymbol(Sym.unscopables) ? it : xit;
   var ifShimIt = (typeof process !== 'undefined' && process.env.NO_ES6_SHIM) ? it.skip : it;
 
+  var isNegativeZero = function (x) {
+    return (1 / x) < 0;
+  };
+
   describe('Array', function () {
     var list = [5, 10, 15, 20];
 
@@ -975,6 +979,12 @@ var runArrayTests = function (it) {
         expect(reduced).to.equal(accumulator);
       });
     });
+
+    describe('#indexOf()', function () {
+      it('converts second argument from -0 to +0', function () {
+        expect(isNegativeZero([true].indexOf(true, -0))).to.equal(false);
+      });
+    });
   });
 };