Missing initializer in const declaration Error in JS and TS

# Table of Contents

1. Missing initializer in const declaration Error in JS
2. 'Const' declarations must be initialized in TypeScript

If you got the error when using TypeScript, click on the second subheading.

# Missing initializer in const declaration Error in JS

The "Missing initializer in const declaration" error occurs when a variable is declared using const, but its value is not initialized on the same line.

To solve the error, initialize the variable on the same line you declare it, e.g. const num = 30;.

Here's an example of how the error occurs.

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// ⛔️ Missing initializer in const declaration
const country;

// ⛔️ Missing initializer in const declaration
const age;

age = 30;

We declared a variable using the const keyword, however, we didn't initialize its value on the same line.

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const name = 'bobby';

const age = 30;

const arr = ['bobby', 'hadz', 'com'];

# Use let to declare variables that can be reassigned

If you need to declare a variable that can be reassigned, use the let statement instead.

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// ✅ let = can be reassigned
let country;
country = 'Chile';

// ✅ const = Set value upon declaration
const age = 30;

Variables declared using let can be reassigned later on in your code.

The error message should display the file in which the error occurred and the specific line.

# Variables declared let cannot be redeclared

Variables declared using the let statement can be reassigned. However, they cannot be redeclared.

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let country;
// ⛔️ Identifier 'country' has already been declared
let country = 'Chile';

We tried to redeclare the country variable and got an error. Instead, omit the let keyword when reassigning a variable declared using let.

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let age;

age = 30;

console.log(age); // 👉️ 30

age = 31;

console.log(age); // 👉️ 31

# Set the value on the same line when using const

If you don't need to reassign the variable, use the const keyword and set its value when declaring the variable.

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// ✅ Works
const country = 'Chile';

// ⛔️ Error: Assignment to constant variable
country = 'Brazil';

Using const lets the reader of your code know that this variable will not be reassigned at a later point.

# The const keyword doesn't make objects and arrays immutable

Note that the const keyword doesn't make objects or arrays immutable, it just prevents you from reassigning the variable.

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// 👇️ can change the contents of an array declared using const
const arr = ['bobby', 'hadz'];

arr[0] = 'xyz';

console.log(arr); // 👉️ [ 'xyz', 'hadz' ]

// ---------------------------------------------

// 👇️ can change the contents of an object declared using const

const obj = {name: 'bobby'};

obj['name'] = 'alice';

console.log(obj); // 👉️ { name: 'alice' }

We are still able to change the contents of an array or an object declared using const, however, we can't redeclare or reassign the variables.

# 'Const' declarations must be initialized in TypeScript

The error "const declarations must be initialized" occurs when we declare a variable using const but don't assign a value to it.

To solve the error, specify a value in the same statement in which the variable is declared or use the let keyword instead.

Here is an example of how the error occurs.

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// ⛔️ 'const' declarations must be initialized.ts(1155)
const arr: string[];

We declared the arr variable using the const keyword but didn't assign a value to it.

To solve the error, we have to assign a value to the const variable in the same statement in which it was declared.

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const arr: string[] = [];

What value you assign to the variable will depend on its type.

# Using the let keyword to solve the error

An alternative solution is to use the let keyword to declare your variable.

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let arr: string[];

arr = ['bobby', 'hadz'];

arr = ['.', 'com'];

console.log(arr); // 👉️ ['.', 'com']

When you use the let keyword, you can reassign the variable as many times as necessary.

If you declare a const variable without a value, you are effectively declaring an empty variable that cannot be reassigned and given a value later on, which must be a mistake.

We use a colon to give the variable a type and an equal sign to assign a value to it.

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const obj: { name: string } = { name: 'Bobby Hadz' };

Note that variables declared using const cannot be reassigned, but they are not immutable.

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const obj: { name: string } = { name: 'Bobby Hadz' };
obj['name'] = 'Carl';

console.log(obj); // 👉️ {name: 'Carl'}

// ⛔️ Error: Assignment to constant variable.
obj = { name: 'Alan' };

The code sample shows that we are able to change the value of an object declared using const, however, trying to reassign the const variable causes an error.

This is because you aren't allowed to reassign or redeclare a variable declared using the const keyword.