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Message
TypeError: "x" is not a function
Error type
What went wrong?
It was 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
orTypedArray
objects: - When working with
Map
andSet
objects:
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 ]