Quick Answer

Fix "x is not a function" errors.

Understanding the Issue

This TypeError occurs when trying to invoke a non-function value, often from incorrect variable references or timing issues.

The Problem

This code demonstrates the issue:

Javascript Error 
const user = { name: "John" };
user.getName(); // Error: getName is not a function
const notAFunction = "hello";
notAFunction(); // Error

The Solution

Here's the corrected code:

Javascript Fixed 
// Solution 1: Check function type
if (typeof user.getName === "function") {
    user.getName();
}

// Solution 2: Add missing method
user.getName = function() { return this.name; };

// Solution 3: Safe invocation pattern
const safeCall = (fn, ...args) => 
    typeof fn === "function" ? fn(...args) : null;

// Solution 4: Handle async loading properly
import("./module.js")
    .then(module => module.init())
    .catch(err => console.error("Failed to load:", err));

Key Takeaways

Always verify function existence before invocation.