Determine if an object is freed.js

/** 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 <steve.hanov@gmail.com>
*/
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
      freeFn();

      // wait for GC to run, at least 10 seconds
      setTimeout( () => {
        const diff = before - performance.memory.usedJSHeapSize;
        resolve({ 
          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}`)
})