ReferenceError — Undefined Variables

TL;DR

Explanation

ReferenceError: x is not defined means x was never declared with var/let/const. This is different from undefined, which means a variable was declared but has no value. Common causes: typos in variable names, accessing variables before declaration (TDZ for let/const), accessing out-of-scope variables, using browser globals (window.something) in Node. typeof never throws ReferenceError — use typeof x !== 'undefined' to safely check if a variable exists without risk of throwing.

Common Misconception

Why It Matters

Common Mistakes

• Checking if (x) when x might not be declared — use if (typeof x !== 'undefined').
• Accessing let/const variables before their declaration in the same scope (TDZ).
• Relying on var hoisting — var is hoisted but initialised as undefined, not the assigned value.

Code Examples

console.log(userName); // ReferenceError: userName is not defined

// TDZ:
console.log(value); // ReferenceError: Cannot access 'value' before initialization
const value = 42;

// Safe existence check:
if (typeof userName !== 'undefined') {
    console.log(userName);
}

// Always declare before use:
const value = 42;
console.log(value); // 42

References

• ↗ https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Errors/Not_defined

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