JavaScript Iterators

Question

I was going through MDN (Mozilla Developer Network) and came across Iterators and generators

So naturally, I tried the snippets of code given in the page on Google Chrome v21. To be specific, this code:

var it = Iterator(lang);
for (var pair in it)
 print(pair); // prints each [key, value] pair in turn

However, the console returns this error message:

ReferenceError: Iterator is not defined

Why's that? Is the Iterator function deprecated or something? Am I missing a point? Thank you for your help and time :-)

• j.w.r: Somewhat related: Google Chrome - javascript version

Answer 1

Arrays have a built in map function that acts like an iterator.

[1,2,3].map(function(input){console.log(input)});

Standard Out:

1
2
3

Worst case you could easily design an iterator object, didn't full test this but if there are any bugs you should be able to quickly get this working.

var Iterator = function(arr){ return {
 index : -1,
 hasNext : function(){ return this.index <= arr.length; },
 hasPrevious: function(){ return this.index > 0; },
 current: function(){ return arr[ this["index"] ]; },
 next : function(){
 if(this.hasNext()){
 this.index = this.index + 1; 
 return this.current();
 } 
 return false;
 },
 previous : function(){
 if(this.hasPrevious()){
 this.index = this.index - 1
 return this.current();
 }
 return false;
 }
} 
};
var iter = Iterator([1,2,3]);
while(iter.hasNext()){
 console.log(iter.next());
}

• zloctb: Maybee hasNext : function(){ return this.index < arr.length-1; }, ?
• zloctb: for object add var arr= Object.keys(arr)

Answer 2

window.Iterator AFAIK only exists in Firefox, not WebKit.

Answer 3

From this thread:

V8 is an implementation of ECMAScript, not JavaScript. The latter is a non-standardized extension of ECMAScript made by Mozilla.

V8 is intended to be plug-in compatible with JSC, the ECMAScript implementation in WebKit/Safari. As such it implements a number of non-standard extensions of ECMAScript that are also in JSC, and most of these are also in Mozilla's JavaScript languages.

There is no plan to add non-standard features that are not in JSC to V8.

Note: JSC stands for JavaScript Core - the WebKit ECMAScript implementation.

• trincot: This is outdated information.

Answer 4

var makeIterator = function (collection, property) {
 var agg = (function (collection) {
 var index = 0;
 var length = collection.length;
 return {
 next: function () {
 var element;
 if (!this.hasNext()) {
 return null;
 }
 element = collection[index][property];
 index = index + 1;
 return element;
 },
 hasNext: function () {
 return index < length;
 },
 rewind: function () {
 index = 0;
 },
 current: function () {
 return collection[index];
 }
 };
 })(collection);
 return agg;
};
var iterator = makeIterator([5,8,4,2]);
console.log(iterator.current())//5
console.log( iterator.next() )
console.log(iterator.current()) //8
console.log(iterator.rewind());
console.log(iterator.current()) //5

Answer 5

It means that Chrome v21 does not support that feature of JavaScript. It's part of the 1.7 spec. Trying this might help for specifying explicitly 1.7 support in Chrome.

Answer 6

For chrome you could use this

var someArray = [1, 5, 7];
var someArrayEntries = someArray.entries();

here is link, which you could find interesting

Answer 7

Iterator was obsolete, but got a new life:

The question refers to a long obsolete implementation of an Iterator function in Firefox back in the days of JavaScript 1.7, which was not part of an ECMA-262 standard at the time.

However, as of ECMAScript 2025, there is now again a global Iterator. It is an abstract constructor/class such that all core JavaScript functions that return an iterator, inherit from Iterator.prototype, and thereby get access to a set of iterator helper methods.

So now you can do:

const res = Array(1000).keys() // Iterator for 0, 1, 2, 3, 4, ...
 .map(n => n*n) // Iterator for 0, 1, 4, 16, 24, ...
 .filter(n => n % 17 == 1) // Iterator for 1, 256, ...
 .toArray(); // Consume iterator into array
console.log(res);