Debugging "TypeError: X is not a function" in JavaScript

Jul 27, 2020

The TypeError: X is not a function error is a common cause of confusion for JavaScript beginners. JavaScript throws this error when you attempt to call a value that isn't a function. For example:

const x = 42;

x(); // Throws 'TypeError: x is not a function'

Most modern JavaScript runtimes are good about formatting this error, so you know what expression you tried to call that isn't a function. For example, another common cause of this error is calling a non-existant method on an object.

const moment = require('moment');

const date = moment('2020-06-01');

// Typo! Throws 'TypeError: date.formt is not a function'

If JavaScript throws this error in code that you wrote, you should double check the code at the line number in the error's stack trace. Odds are, you either typo-ed, or you need to add a check to make sure the value that you're calling is a function.

// Ensure that `x` is a function to avoid TypeError.
if (typeof x !== 'function') {


A Note on Semicolons

If you don't use semicolons, you can get some surprising instances of this error. For example, the below code throws TypeError: arr[0] is not a function:

const arr = []

const val = arr[0]
(function() { console.log(val) })()

Because there isn't a semicolon at the end of arr[0], JavaScript treats the above code as equivalent to:

const arr = []

const val = arr[0](function() { console.log(val) })()

If you write JavaScript without semicolons, be sure to use a linter to avoid mistakes like this!

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