What are the benefits of using Array.prototype.forEach.call(array, cb) instead of array.forEach(cb)?

Question

What are the benefits of using Array.prototype.forEach.call(array, cb) instead of array.forEach(cb)?

After revisiting some snapshots from the recent ng-europe conference, I stumbled upon a slide that appears to showcase snippets of code from Angular 2. You can view it here:

(Source: )

One thing that confuses me is this:

What could be the reason behind the use of

Array.prototype.forEach.call(array, cb)

instead of the shorter and seemingly equivalent version array.forEach(cb)? My assumption leans towards potential performance impacts.

Is there any other explanation for this discrepancy? Or could my theory about performance be accurate?

javascript angularjs foreach

Answer 1

Answer №1

There are several types of objects that resemble arrays but are not truly arrays. Some examples include:

arguments
children and childNodes collections
NodeList collections returned by methods like document.getElementsByClassName and document.querySelectorAll
jQuery collections
and even strings.

Many array prototype methods are designed to be generic, meaning they can be called on objects that "look" like arrays but are not instances of the Array constructor. This is because these objects have numeric keys and a length property, making them behave like arrays in some contexts.

Here's a simple example demonstrating how you can use Array.prototype.join on a custom array-like object:

Array.prototype.join.call({0: 'one', 1: 'two', length: 2}, ' ');

The output of the above code will be "one two", even though the supplied object is not technically an array.

Answer 2

There are several types of objects that resemble arrays but are not truly arrays. Some examples include:

arguments
children and childNodes collections
NodeList collections returned by methods like document.getElementsByClassName and document.querySelectorAll
jQuery collections
and even strings.

Many array prototype methods are designed to be generic, meaning they can be called on objects that "look" like arrays but are not instances of the Array constructor. This is because these objects have numeric keys and a length property, making them behave like arrays in some contexts.

Here's a simple example demonstrating how you can use Array.prototype.join on a custom array-like object:

Array.prototype.join.call({0: 'one', 1: 'two', length: 2}, ' ');

The output of the above code will be "one two", even though the supplied object is not technically an array.

Answer 3

Answer №2

The code is exhibiting a defensive behavior when it comes to element.attributes or element.children, especially if they are not in array format.

Answer 4

The code is exhibiting a defensive behavior when it comes to element.attributes or element.children, especially if they are not in array format.

What are the benefits of using Array.prototype.forEach.call(array, cb) instead of array.forEach(cb)?

One thing that confuses me is this:

Answer №1

Answer №2

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