Why is there an empty array in JavaScript\'s [].forEach.call() function?

Understanding the [].forEach.call() Function in JavaScript

Are you curious about the intriguing use of [].forEach.call() in JavaScript? Let's delve into its purpose and functionality.

In JavaScript, arrays possess a powerful method called forEach, which allows you to perform operations on each element of an array. However, in some instances, you may encounter code where an empty array is employed prior to applying forEach to a node list.

Breaking Down the Code

Consider the following code snippet:

[].forEach.call(document.querySelectorAll('a'), function(el) {
  // Perform some operation on the current node
});

What's happening here? Let's break it down:

• []: This is an empty array. It serves no actual purpose in the code. Its presence is merely to provide access to the forEach method, which is a prototype of arrays.
• forEach: The forEach method iterates over each element in an array (or array-like object) and calls the specified function with three arguments: the element, its index, and the array itself.
• .call: The .call method is used to invoke a function with a specified value for its "this" keyword. In this case, it's changing the "this" value of the forEach method from the empty array to the list of tags. This allows the function to access each element by index.

Example Usage

Let's look at an example to better understand how this code works:

const myNodeList = document.querySelectorAll('a');
[].forEach.call(myNodeList, function(el) {
  el.style.color = 'blue';
});

In this code:

• We obtain a node list of all tags on the page.
• The [].forEach.call() method changes the "this" value to myNodeList, and invokes the forEach method over the elements.
• Within the callback function, we can access each element using el and modify its style (in this case, changing its color to blue).

Alternative Syntax for ES6

In ES6 (ECMAScript 2015) and beyond, a more modern and concise syntax exists for achieving the same result:

[...document.querySelectorAll('a')].forEach(el => {
  el.style.color = 'blue';
});

In this updated syntax, we use the spread operator (...) to convert the node list into an array, which can then be iterated over using forEach.

Conclusion

The [].forEach.call() technique provides a concise and effective way to apply the forEach method to array-like objects such as node lists. By utilizing this technique, you can easily perform operations on each element in the list. Remember, however, that in modern JavaScript, alternative syntaxes such as the spread operator may offer a more elegant solution.

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