How to Fix TypeError: is not a function in JavaScript

The TypeError: is not a function happens when you try to call something as a function that isn't one. To fix it, check that the variable you call is actually a function and not undefined, null, or another type like a string or object.

Why This Happens

This error occurs when JavaScript expects a function but finds a different type instead. For example, if you try to call a variable that holds a string or an object as if it were a function, JavaScript throws this error.

javascript

const greet = "hello";
greet();

Output

TypeError: greet is not a function

The Fix

Make sure the variable you call is actually a function. If you want to call a function, assign a function to the variable. If you accidentally overwrote a function with another type, fix that.

javascript

const greet = function() {
  console.log("hello");
};
greet();

Output

hello

Prevention

To avoid this error, always check your variables before calling them. Use clear names and keep functions separate from other data. Tools like linters can warn you if you try to call something that is not a function.

• Use typeof variable === 'function' to check before calling.
• Keep your code organized to avoid overwriting functions.
• Use modern editors with linting to catch mistakes early.

Related Errors

Other common errors include:

• TypeError: undefined is not a function - calling a variable that is undefined.
• TypeError: null is not a function - calling a variable that is null.
• ReferenceError: variable is not defined - using a variable that does not exist.

Fixes usually involve checking variable definitions and types before calling.

Key Takeaways

Always ensure the variable you call is a function before invoking it.

Check variable types with typeof to prevent calling non-functions.

Use clear naming and avoid overwriting functions with other values.

Use linters and modern editors to catch these errors early.

Understand related errors like calling undefined or null as functions.