Lessons from open-source: Convert an array-like HTML NodeList to a standard javascript array.

This lesson is picked from next.js/packages/next. In this article, you will learn how to convert an array-like HTML Nodelist into a standard javascript array.

looseToArray Function:

The following code snippet is from https://github.com/vercel/next.js/blob/canary/packages/next/src/client/index.tsx#L78C1-L79C1

const looseToArray = <T extends {}>(input: any): T[] => [].slice.call(input)

looseToArray is a generic function that takes an input of type any.
It’s using the slice method on an empty array ([]) and calling it with input as its context ([].slice.call(input)).
The purpose is to convert something array-like (such as a NodeList) into a real array.

Notice the “any” type for input parameter. There are quite some places where “any” type is used in nextjs source code. I used to think “any” should be avoided at all costs, but it’s okay no code is too perfect.

const currentStyleTags: HTMLStyleElement[] = looseToArray<HTMLStyleElement>(
 document.querySelectorAll('style[data-n-href]')
)

The above snippet is picked from onStart function inside next/client.

document.querySelectorAll(‘style[data-n-href]’) selects all <style> elements with a data-n-href attribute in the document.
The result of this query is a NodeList, which is array-like but not a real array.
looseToArray<HTMLStyleElement> is used to convert the NodeList into a real array of HTMLStyleElement.

So, the looseToArray function is used here to convert the NodeList obtained from document.querySelectorAll into a standard JavaScript array. The purpose of using slice.call is a common technique to convert array-like objects into arrays because the slice method can be called on array-like objects, and it returns a new array.

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Experiment

You can execute the following code to find out how HTML NodeList looks like after converting it to an array.

const looseToArray = function(input) {
  return [].slice.call(input);
};

const currentStyleTags = looseToArray(
  document.querySelectorAll('.pw-post-body-paragraph')
);

console.log("document.querySelectorAll('.pw-post-body-paragraph')", 
  document.querySelectorAll('.pw-post-body-paragraph'))

console.log("currentStyleTags", currentStyleTags);

You need to change document.querySelectorAll(‘style[data-n-href]’) to the relevant DOM selector function that finds an element in the browser.

Array-like NodeList

NodeList converted to array

Conclusion:

In JavaScript, the NodeList returned by document.querySelectorAll is not a true array but rather a collection of nodes. While it looks like an array, it lacks many of the array methods and properties. The looseToArray function in nextjs source code is used to convert the NodeList into a regular array.