ReferenceError

ReferenceError is thrown when code references a variable that doesn't exist in the current scope. This happens when using a variable before it's declared, or when the variable name is misspelled.

Common causes

• Using a variable before it is declared (with let/const)
• Typo in a variable or function name
• Accessing a variable from the wrong scope
• Forgetting to import a module or library
• Using a browser API in a non-browser environment (e.g., window in Node.js)

How to fix it

• Declare variables before using them
• Check for typos in variable and function names
• Use typeof to safely check if a variable exists: typeof x !== "undefined"
• Ensure imports are present for external modules
• Use try/catch when accessing environment-specific globals

Example

// Error example
console.log(myVariable);  // ReferenceError: myVariable is not defined

// Temporal dead zone
console.log(x);  // ReferenceError: Cannot access 'x' before initialization
let x = 10;

// Fix by declaring first
let myVariable = "hello";
console.log(myVariable);  // "hello"

// Safe check for existence
if (typeof myVariable !== "undefined") {
  console.log(myVariable);
}

Related errors