Fix - Cannot access before initialization Error in JS

Fix - Cannot access before initialization #

The "Cannot access before initialization" error occurs when a variable declared using let or const is accessed before it was initialized in the scope. To solve the error, make sure to initialize the variable before accessing it.

Here are some examples of how the error occurs.

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arr = [3, 2, 1]; // ⛔️ Cannot access 'arr' before initialization
let arr = [1, 2, 3];

if (5 === 5) {
  console.log(num); // ⛔️ Cannot access 'num' before initialization
  const num = 100;
}

function example() {
  console.log(str); // ⛔️ Cannot access 'str' before initialization

  let str = 'hello';
}

To solve the error, we have to initialize the variables before using them.

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let arr = [1, 2, 3];
arr = [3, 2, 1];

if (5 === 5) {
  const num = 100;
  console.log(num);
}

function example() {
  let str = 'hello';
  console.log(str);
}

If you're unable to track down where exactly the error occurs in your code, look at the error message in your browser's console or your Node.js terminal.

The screenshot above shows that the error was thrown on line 4 in the index.js file.

If a variable is declared using the var keyword the declaration gets hoisted to the top. However, this is not the case when using the let and const keywords.

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console.log(arr); // 👉️ undefined

var arr = [1, 2, 3];

Had we used let or const, we would have gotten the "Cannot access before initialization" error.

How hoisting works with variables declared using var is it lifts the variable declaration to the top of the file.

Behind the scenes, this is what happens.

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var arr;

console.log(arr); // 👉️ undefined

arr = [1, 2, 3];

However, this is not the behavior when using the let and const keywords to declare a variable.

When using let and const always make sure to initialize the variables before accessing them.

Further Reading #

• Check if a Variable is Defined or Initialized in JavaScript
• Check if Variable is Null or Undefined in JavaScript
• Check if a Variable is a String using JavaScript
• Check if Variable Doesn't Equal Multiple Values in JS
• Concatenate a String with a Variable in JavaScript
• Reassign a Variable if Null or Undefined using JavaScript
• Set a Variable's Value if it's Undefined using JavaScript
• Interpolate Variable in a String in JavaScript