TypeError

TypeError is thrown when an operation could not be performed, typically when a value is not of the expected type. This includes calling non-functions, accessing properties of null/undefined, and using operators on incompatible types.

Common causes

• Calling a non-function value as a function
• Accessing a property of null or undefined
• Passing the wrong type of argument to a built-in method
• Using an operator on an incompatible type (e.g., spreading a non-iterable)
• Assigning to a read-only property in strict mode

How to fix it

• Use optional chaining (?.) to safely access nested properties
• Check for null/undefined before accessing properties
• Validate function arguments with typeof checks
• Use TypeScript or JSDoc to catch type errors at compile time
• Add runtime type guards before operations

Example

// Error example
const user = null;
console.log(user.name);  // TypeError: Cannot read properties of null

// Fix with optional chaining
console.log(user?.name);  // undefined (no error)

// Fix with guard check
if (user != null) {
  console.log(user.name);
}

Related errors