Quick Answer

Resolve ReferenceError when using undefined variables.

Understanding the Issue

This error occurs when trying to access a variable that hasn't been declared in the current scope. In strict mode, this also occurs for undeclared variables that would otherwise become global properties.

The Problem

This code demonstrates the issue:

Javascript Error 
console.log(x); // x was never declared
function example() {
    console.log(y); // y not defined in this scope
}

The Solution

Here's the corrected code:

Javascript Fixed 
// Solution 1: Declare before use
const x = 5;
console.log(x);

// Solution 2: Proper scoping
function example() {
    const y = 10;
    console.log(y);
}

// Solution 3: Check typeof
if (typeof z !== 'undefined') {
    console.log(z);
}

Key Takeaways

Always declare variables before use and understand JavaScript scoping rules.