Confused about polyfill of Array.prototype.reduce

Question

This code is polyfill of Array.prototype.reduce given on Mozilla Developer Network. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce

// Production steps of ECMA-262, Edition 5, 15.4.4.19
// Reference: http://es5.github.io/#x15.4.4.19
if (!Array.prototype.map) {
  Array.prototype.map = function(callback /*, thisArg*/) {
    var T, A, k
    if (this == null) {
      throw new TypeError('this is null or not defined')
    }

    var O = Object(this)
    var len = O.length >>> 0

    if (typeof callback !== 'function') {
      throw new TypeError(callback + ' is not a function')
    }

    if (arguments.length > 1) {
      T = arguments[1]
    }

    A = new Array(len)
    k = 0

    while (k < len) {
      var kValue, mappedValue

      if (k in O) {
        kValue = O[k]
        mappedValue = callback.call(T, kValue, k, O)
        A[k] = mappedValue
      }
      k++
    }

    return A
  }
}

What I don't understand is these two lines

1.Why not just use this?

var O = Object(this)

2.Is it possible this be null, why need this code below?

if (this == null) {
  throw new TypeError('this is null or not defined')
}

3.Why we need k in O? while k < len, k always in O, is it useless condition?

if (k in O) {
    kValue = O[k]
    mappedValue = callback.call(T, kValue, k, O)
    A[k] = mappedValue
  }

I am going to comment this since I'm not 100% sure. **1:** `var O` wraps `this` as an `object`. **2:** `this` can be null as you can your input as null. **3:** `Objects created from built–in constructors like Array and Object have inherited non–enumerable properties from Object.prototype` taken from [`for...in statement`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...in) — Alex, Jul 09 '18 at 11:22
Just curious, why does your title ask about `reduce` but your question body shows the polyfill for `map`? — Bergi, Jul 09 '18 at 12:55
"*while `k < len`, `k` always in `O`*" - no, not in [sparse arrays](https://stackoverflow.com/questions/1510778/are-javascript-arrays-sparse) (or any other object `O` that has a `length` without all the index properties) — Bergi, Jul 09 '18 at 13:12

Sotiris Kiritsis · Accepted Answer · 2018-07-10T05:51:04.617

1.Why not just use this?

var O = Object(this)

In Edition 3, an undefined or null thisArg is replaced with the global object and ToObject is applied to all other values and that result is passed as the this value.

So my guess is that the reasons this is being done is to keep consistency.

2.Is it possible this be null, why need this code below?

if (this == null) {
  throw new TypeError('this is null or not defined')
}

Yes, it is possible for this to be null.

Execute the following code with and without strict mode. You will notice that in strict mode the output is null, while in the other case its the window.

// "use strict";
var x = function(){
    console.log(this);
    return this;
}

x.call(null);

3.Why we need k in O? while k < len, k always in O, is it useless condition?

if (k in O) {
    kValue = O[k]
    mappedValue = callback.call(T, kValue, k, O)
    A[k] = mappedValue
  }

Not a useless condition, because it checks if a property exists.(see in operator examples)

Adding an example with sparse arrays:

var a = new Array(3);
console.log(a); // [ , , ]
console.log(a.length); // 3
console.log(a[0]); // undefined

No, `in` does not check for enumerability. It checks for existence. — Bergi, Jul 09 '18 at 12:54
@Sotiris Kiritsis About 3. I thought while `k < len`, then k always in O, so k must exist in O — Vic, Jul 10 '18 at 01:54
Updated point 3, with an additional example. See @Bergi comment above as well. — Sotiris Kiritsis, Jul 10 '18 at 05:52

Confused about polyfill of Array.prototype.reduce

1 Answers1