ReferenceError: x is not defined

JavaScript could not find that name in any scope. A typo, a let/const used outside its block, or a browser-only global used on the server.

JavaScriptScope and closure bugs

What it means

Unlike reading a missing property (which gives undefined), using a name that was never declared throws. Common variants have very specific causes: "document is not defined" and "window is not defined" mean browser code ran in Node or during server-side rendering; "require is not defined" means CommonJS code ran as an ES module or in the browser.

Common causes

1. A variable declared inside a block

let and const exist only inside the { } where they were declared.

Breaks

if (user) {
  const name = user.name;
}
console.log(name);

Works

let name = "";
if (user) {
  name = user.name;
}
console.log(name);

2. document or window on the server

Next.js and other frameworks render on the server first, where there is no DOM.

Breaks

const width = window.innerWidth;   // top level of a component

Works

useEffect(() => {
  setWidth(window.innerWidth);   // runs in the browser only
}, []);

3. require in an ES module

Files with "type": "module" or .mjs use import, and require does not exist.

Breaks

const fs = require("fs");

Works

import fs from "fs";

4. A typo

userName and username are different names.

Breaks

const userName = "ada";
console.log(username);

Works

const userName = "ada";
console.log(userName);

How to find it in your own code

Read the name in the message and search for its declaration. If there is none, it is a typo or a missing import. If it is inside { }, move the declaration out. If it is document, window or localStorage, the code is running somewhere without a browser — move it into an effect or event handler.

Try it: fix this bug

medium

Calling findFirstNegative with any array throws: ReferenceError: result is not defined.

function findFirstNegative(nums) {
    for (let i = 0; i < nums.length; i++) {
        if (nums[i] < 0) {
            let result = nums[i];
        }
    }
    return result;
}

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Other common errors

ReferenceError: Cannot access before initialization TypeError: Assignment to constant variable.UnboundLocalError: cannot access local variable NameError: name 'x' is not defined TypeError: 'list' object is not callable TypeError: Cannot read properties of undefined [object Promise] in output TypeError: Cannot read properties of undefined (reading 'map')