Argument type not assignable TypeScript TS2345

Why This Happens

In TypeScript, a function argument has the wrong type. This is one of the most common TypeScript errors developers encounter. Understanding the root cause helps you fix it quickly and prevent it from recurring.

The Problem

function add(a: number, b: number) {}
add('1', 2);

The Fix

function add(a: number, b: number) {}
add(1, 2);

Step-by-Step Fix

1. 1

   Identify the error

   Look at the error message: TS2345: Argument of type 'string' is not assignable. This tells you exactly what went wrong.

2. 2

   Find the cause

   Check the stack trace to find which line of your TypeScript code triggered this error.

3. 3

   Apply the fix

   Use the corrected code pattern shown above. Test to confirm the error is resolved.