重新定义的 toString 的 getOwnPropertyDescriptor 应该是未定义的

Question

I have redefined toString of HTMLCanvasElement.prototype.toDataURL . When I am getting property descriptor of toString it should return undefined , but it returns the function. Any ideas how to fix it?

You can execute code here https://jsfiddle.net/nqk50a8r/

Object.defineProperty(HTMLCanvasElement.prototype.toDataURL, 'toString', {
    value: function () { return 'function toDataURL() { [native code] }';}
});

var desc = Object.getOwnPropertyDescriptor(HTMLCanvasElement.prototype.toDataURL, 'toString');

console.log(desc === undefined);

If you removed defineProperty block you will see that it returns undefined .

I redefined toDataURL by next code:

Object.defineProperty(HTMLCanvasElement.prototype, 'toDataURL', {
    value: function () { return 'new valu' }
});

If I didn't redefine toString it will return code itself when call toString .

Answer 1

HTMLCanvasElement.prototype.toDataURL does not have an own property toString by default. The toString method you get by referencing HTMLCanvasElement.prototype.toDataURL.toString is prototypally inherited from Function.prototype.toString :

 console.log( HTMLCanvasElement.prototype.toDataURL.toString === Function.prototype.toString );

No property exists directly on HTMLCanvasElement.prototype.toDataURL.toString . But if you add one yourself, via

Object.defineProperty(HTMLCanvasElement.prototype.toDataURL, 'toString',

then it will have such an own property, and will also log a property descriptor if you examine it.

If you want to monkeypatch your custom toString method while keeping HTMLCanvasElement.prototype.toDataURL.toString empty, you can overwrite Function.prototype.toString .

Note that while this is technically possible, it's also very strange to do, and a bad idea to mutate built-in prototypes:

const origToString = Function.prototype.toString;
Function.prototype.toString = function() {
  if (this === HTMLCanvasElement.prototype.toDataURL) return 'function toDataURL() { [native code] }';
  else return origToString.call(this);
}