X.prototype.y called on incompatible type

Message

TypeError: Function.prototype.toString called on incompatible object (Firefox)
TypeError: Function.prototype.bind called on incompatible target (Firefox)
TypeError: Method Set.prototype.add called on incompatible receiver undefined (Chrome)
TypeError: Bind must be called on a function (Chrome)

Error type

TypeError

What went wrong?

When this error is thrown, a function (on a given object), is called with a this not corresponding to the type expected by the function.

This issue can arise when using the Function.prototype.call() or Function.prototype.apply() methods, and providing a this argument which does not have the expected type.

This issue can also happen when providing a function (stored in an object) as an argument to another function. In this case, the object won't the this target of the function. To work-around this issue, you will either need to provide a lambda which is making the call, or use the Function.prototype.bind() function to force the this argument to the expected object.

Examples

Invalid cases

var mySet = new Set;
['bar', 'baz'].forEach(mySet.add);
// mySet.add is a function, but "mySet" is not captured as this.

var myFun = function () {};
['bar', 'baz'].forEach(myFun.bind);
// myFun.bind is a function, but "myFun" is not captured as this.

Valid cases

var mySet = new Set;
['bar', 'baz'].forEach(mySet.add.bind(mySet));
// This works due to binding "mySet" as this.

var myFun = function () {};
['bar', 'baz'].forEach(x => myFun.bind(x));
// This works using the "bind" function. It creates a lambda forwarding the argument.

See also

• Function.prototype.call()
• Function.prototype.apply()
• Function.prototype.bind()