Pen Settings



CSS Base

Vendor Prefixing

Add External Stylesheets/Pens

Any URLs added here will be added as <link>s in order, and before the CSS in the editor. You can use the CSS from another Pen by using its URL and the proper URL extension.

+ add another resource


Babel includes JSX processing.

Add External Scripts/Pens

Any URL's added here will be added as <script>s in order, and run before the JavaScript in the editor. You can use the URL of any other Pen and it will include the JavaScript from that Pen.

+ add another resource


Add Packages

Search for and use JavaScript packages from npm here. By selecting a package, an import statement will be added to the top of the JavaScript editor for this package.


Save Automatically?

If active, Pens will autosave every 30 seconds after being saved once.

Auto-Updating Preview

If enabled, the preview panel updates automatically as you code. If disabled, use the "Run" button to update.

Format on Save

If enabled, your code will be formatted when you actively save your Pen. Note: your code becomes un-folded during formatting.

Editor Settings

Code Indentation

Want to change your Syntax Highlighting theme, Fonts and more?

Visit your global Editor Settings.


                <div>Hello there</div>




                /** Determines if an object is freed
@param obj is the object of interest
@param freeFn is a function that frees the object.
@returns a promise that resolves to {freed: boolean, memoryDiff:number}

@author Steve Hanov <[email protected]>
function isObjectFreed(obj, freeFn) {
  return new Promise( (resolve) => {
    if (!performance.memory) {
      throw new Error("Browser not supported.");

    // When obj is GC'd, the large array will also be GCd and the impact will
    // be noticeable.
    const allocSize = 1024*1024*1024;
    const wm = new WeakMap([[obj, new Uint8Array(allocSize)]]);

    // wait for memory counter to update
    setTimeout( () => {
      const before = performance.memory.usedJSHeapSize;

      // Free the memory

      // wait for GC to run, at least 10 seconds
      setTimeout( () => {
        const diff = before - performance.memory.usedJSHeapSize;
          freed: diff >= allocSize, 
          memoryDiff: diff - allocSize
      }, 10000);
    }, 100);

let foo = {bar:1};

isObjectFreed(foo, () => foo = null).then( (result) => {
  document.write(`Object GCd:${result.freed}, ${result.memoryDiff} bytes freed`)
}, (error) => {
  document.write(`Error: ${error.message}`)