JavaScript

X is not defined (ReferenceError)

You used a name that doesn't exist in scope.

What it looks like

Uncaught ReferenceError: $ is not defined
    at main.js:3:1

What does "X is not defined (ReferenceError)" mean?

Thrown when code uses a name that does not exist anywhere in scope. The classic case is a library such as jQuery being used before its script tag has loaded, or a variable declared inside a function being used outside it.

What causes "X is not defined (ReferenceError)"?

• A typo in a variable/function name.
• Using a variable before it's declared.
• A missing import or script not loaded.
• Wrong scope (defined inside another function/block).

How do I confirm the cause?

1. Search the codebase for where the name is declared — it may not exist at all, or be spelled differently.
2. For a library global, check the script tag order and whether a defer, async or failed download means it is not loaded yet.
3. For let and const, check the variable is not being used above its declaration in the same block.

How do I fix "X is not defined (ReferenceError)"?

• Check the spelling of the name.
• Make sure it's declared/imported and the script is loaded first.
• Move the declaration above its use.

How do I stop it happening again?

• Use modules and explicit imports instead of relying on globals from script tags.
• Run a linter such as ESLint with no-undef, which flags undeclared names as you type.

Got a different error? Paste it into the Explain This Error tool → Identifies the family and the fix — runs locally, nothing uploaded.

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