TypeError: "x" is not a function. (V8-based & Firefox & Safari)

It attempted to call a value from 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 the JavaScript Array object does.

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()

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

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

The correct function name is getElementById:

const x = document.getElementById("foo");

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.

const 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:

const numbers = [1, 4, 9];

numbers.map(function (num) {
  return num * 2;
}); // [2, 8, 18]

Sometimes when making a class, you may have a property and a function with the same name. Upon calling the function, the compiler thinks that the function ceases to exist.

function Dog() {
  this.age = 11;
  this.color = "black";
  this.name = "Ralph";
  return this;
}

Dog.prototype.name = function (name) {
  this.name = name;
  return this;
};

const myNewDog = new Dog();
myNewDog.name("Cassidy"); //Uncaught TypeError: myNewDog.name is not a function

Use a different property name instead:

function Dog() {
  this.age = 11;
  this.color = "black";
  this.dogName = "Ralph"; //Using this.dogName instead of .name
  return this;
}

Dog.prototype.name = function (name) {
  this.dogName = name;
  return this;
};

const myNewDog = new Dog();
myNewDog.name("Cassidy"); //Dog { age: 11, color: 'black', dogName: 'Cassidy' }

In math, you can write 2 × (3 + 5) as 2*(3 + 5) or just 2(3 + 5).

Using the latter will throw an error:

const sixteen = 2(3 + 5);
console.log(`2 x (3 + 5) is ${sixteen}`);
// Uncaught TypeError: 2 is not a function

You can correct the code by adding a * operator:

const sixteen = 2 * (3 + 5);
console.log(`2 x (3 + 5) is ${sixteen}`);
// 2 x (3 + 5) is 16

Ensure you are importing the module correctly.

An example helpers library (helpers.js)

const helpers = function () {};

helpers.groupBy = function (objectArray, property) {
  return objectArray.reduce((acc, obj) => {
    const key = obj[property];
    acc[key] ??= [];
    acc[key].push(obj);
    return acc;
  }, {});
};

export default helpers;

The correct import usage (App.js):

import helpers from "./helpers";

• Functions