JSTests/test262/test/built-ins/Array/prototype/filter/15.4.4.20-9-c-iii-1-6.js

// Copyright (c) 2012 Ecma International.  All rights reserved.
// This code is governed by the BSD license found in the LICENSE file.

/*---
esid: sec-array.prototype.filter
es5id: 15.4.4.20-9-c-iii-1-6
description: >
    Array.prototype.filter - values of 'to' are accessed during each
    iteration when 'selected' is converted to true and not prior to
    starting the loop
---*/

        var toIndex = [];
        var called = 0;

        //By below way, we could verify that 'to' would be setted as 0, 1, ..., length - 1 in order, and each value will be setted one time.
        function callbackfn(val, idx, obj) {
            called++;
            //Each position should be visited one time, which means 'to' is accessed one time during iterations.
            if (toIndex[idx] === undefined) {
                //when current position is visited, its previous index should has been visited.
                if (idx !== 0 && toIndex[idx - 1] === undefined) {
                    return false;
                }
                toIndex[idx] = 1;
                return true;
            } else {
                return false;
            }
        }
        var newArr = [11, 12, 13, 14].filter(callbackfn, undefined);

assert.sameValue(newArr.length, 4, 'newArr.length');
assert.sameValue(called, 4, 'called');