Errors: Not a function

Message

TypeError: "x" is not a function

Error type

TypeError.

What went wrong?

It attempted to call a value like a function, but the value is not actually a function. Some code expects you to provide a function, but that didn't happen.

Maybe there is a typo in the function name? Maybe the object you are calling the method on does not have this function? For example, JavaScript objects have no map function, but JavaScript Array object do.

There are many built-in functions in need of a (callback) function. You will have to provide a function in order to have these methods working properly:

• When working with Array or TypedArray objects:
  • Array.prototype.every(), Array.prototype.some(), Array.prototype.forEach(), Array.prototype.map(), Array.prototype.filter(), Array.prototype.reduce(), Array.prototype.reduceRight(), Array.prototype.find()
• When working with Map and Set objects:
  • Map.prototype.forEach() and Set.prototype.forEach()

Examples

A typo in the function name

In this case, which happens way too often, there is a typo in the method name:

var x = document.getElementByID('foo');
// TypeError: document.getElementByID is not a function

The correct function name is getElementById:

var x = document.getElementById('foo');

Function called on the wrong object

For certain methods, you have to provide a (callback) function and it will work on specific objects only. In this example, Array.prototype.map() is used, which will work with Array objects only.

var obj = {a: 13, b: 37, c: 42};

obj.map(function(num) {
  return num * 2;
});

// TypeError: obj.map is not a function

Use an array instead:

var numbers = [1, 4, 9];

numbers.map(function(num) { 
  return num * 2; 
}); 

// Array [2, 8, 18]

See also

• Functions