Determining if an object is a Map in JavaScript [duplicate]

Question

I am writing a function that returns true if the argument passed to it is an instance of a JavaScript Map.

As you may have guessed typeof new Map() returns the string object and we don't get a handy Map.isMap method.

Here is what I have so far:

function isMap(v) {
  return typeof Map !== 'undefined' &&
    // gaurd for maps that were created in another window context
    Map.prototype.toString.call(v) === '[object Map]' ||
    // gaurd against toString being overridden
    v instanceof Map;
}

(function test() {
  const map = new Map();

  write(isMap(map));

  Map.prototype.toString = function myToString() {
    return 'something else';
  };

  write(isMap(map));
}());

function write(value) {
  document.write(`${value}<br />`);
}

So far so good, but when testing maps between frames and when toString() has been overridden, isMap fails (I do understand why).

For Example:

<iframe id="testFrame"></iframe>
<script>
  const testWindow = document.querySelector('#testFrame').contentWindow;
  // false when toString is overridden 
  write(isMap(new testWindow.Map())); 
</script>

Here is a full Code Pen Demonstrating the issue

Is there a way to write the isMap function so that it will return true when both toString is overridden and the map object originates from another frame?

Answer 1

You can check Object.prototype.toString.call(new testWindow.Map).

If that has been overridden, you're probably out of luck.