Quick Answer

Variable or function is not declared, misspelled, or out of scope. Check spelling, declare variables with let/const, and verify scope access.

Understanding the Issue

ReferenceError is one of the most common JavaScript errors, occurring when the engine cannot find a reference to a variable, function, or object. This happens due to typos in variable names, accessing variables before declaration (temporal dead zone), scope issues, or missing imports. Understanding variable hoisting, scope chains, and proper declaration patterns helps prevent these errors.

The Problem

This code demonstrates the issue:

Javascript Error 
// Problem 1: Undeclared variable
function calculateTotal() {
    let price = 100;
    let tax = 0.08;
    // Typo in variable name
    return prise * (1 + tax); // ReferenceError: prise is not defined
}

// Problem 2: Accessing before declaration
function processData() {
    console.log(result); // ReferenceError: Cannot access before initialization
    let result = "processed";
    return result;
}

calculateTotal();
processData();

The Solution

Here's the corrected code:

Javascript Fixed 
// Solution 1: Fix typos and declare variables
function calculateTotal() {
    let price = 100;
    let tax = 0.08;
    // Fixed typo: prise -> price
    return price * (1 + tax);
}

console.log("Total:", calculateTotal()); // Works correctly

// Solution 2: Declare before use
function processData() {
    let result = "processed"; // Declare first
    console.log(result); // Now accessible
    return result;
}

console.log("Data:", processData());

// Solution 3: Proper scope management
let sharedVar = "shared"; // Global scope

function outerFunction() {
    let localVar = "local";
    
    function innerFunction() {
        console.log(localVar); // Accessible in inner scope
        console.log(sharedVar); // Accessible from global
    }
    
    innerFunction();
    return localVar;
}

console.log("Outer result:", outerFunction());

// Solution 4: Safe variable access
function safeAccess() {
    // Check if variable exists
    if (typeof someVariable !== "undefined") {
        console.log("Variable exists:", someVariable);
    } else {
        console.log("Variable not defined, using default");
        let someVariable = "default value";
        console.log("Default:", someVariable);
    }
}

safeAccess();

Key Takeaways

Always declare variables before use with let/const. Check for typos in variable names. Understand scope rules and variable hoisting. Use typeof checks for optional variables.