How to load third-party Javascript on demand

1 min read

A small script to dynamically (and, asynchronously) load third-party Javascript files on demand.

  • It inserts a <script> tag in the head to load the script.
  • Checks if a <script> tag already exists with the same source to avoid duplicate script tags.
    • I use it with a Web Component based library where another component may already have loaded the script.
  • Callbacks after successful or failed load.
  • Note: I also use a retry logic (that removes and re-attaches the script tag) to avoid temporary network failures. I have not included it here for simplicity.
* @param {string} src The URL of the script.
* @callback onLoadSuccessCallback Callback when script is loaded successfully.
* @callback onLoadErrorCallback Callback when the script fails to load.

function loadScript(src, onLoadSuccessCallback, onLoadErrorCallback) {
if (!src) {
console.error("Error: missing src");

// Check if the script is already loaded
if (document.querySelector('script[src="' + src + '"]')) {
// Script already loaded...
console.warn("Script Already Loaded. Skipping: ", src);
} else {
// Script not already leaded; load script...
// Create script tag
const js = document.createElement('script');
js.src = src;
js.setAttribute("async", "");

// Setup success callback
if (onLoadSuccessCallback) {
js.onload = onLoadSuccessCallback;

// Setup error callback
if (onLoadErrorCallback) {
js.onerror = onLoadErrorCallback;

// Add the script tag to <head>

Use it like this:

loadScript('url/or/path/to/script.js', () => {
console.log("Script loaded");
// Do something with the script.
}, () => {
console.error("Script load failed");
// Handle error.
All Notes Suggest Edit