CODEWITHSUNDEEP
javascriptconstants
Aditeya Pandey
Aditeya Pandey

Reputation: 671

Declare a const variable in javascript without initialization

In javascript, as per my understanding, we can declare variables with let and var keywords and assign it a value later. 

var a;
a = 5;

But, when I try the same approach with const keyword it does not work. 

const a;
Uncaught SyntaxError: Missing initializer in const declaration

I understand my question has a basic contradiction because const keywords are used for storing immutable variables. Therefore, a good coding practice may require it to be initialized while being declared.

However, I want to store global value in a const variable that is being fetched by an API. So, I was wondering if there is a way to assign a value to a pre-declared const variable in javascript?

Upvotes: 6

Views: 29693

Answers (4)

Barmar
Barmar

Reputation: 780798

const applies to the variable, not the value. You have to initialize them, because you can't assign them later -- that's the only difference they have. You can't have a variable that you can assign just once, but not reassign.

If the result is an object, you can declare the constant to hold an empty object, and then use Object.assign() to fill it in with the API response.

const result = {};
fetch(...).then(response => response.json()).then(data => Object.assign(result, data));

Upvotes: 4

Amit Sharma
Amit Sharma

Reputation: 1795

I think you can not do that way . In JavaScript const variables must be assigned a value when they are declared:

Incorrect

const PI;
PI = 3.14159265359;

Correct

const PI = 3.14159265359;

For more reference you can take a look at this which gives you a better explanation

https://www.w3schools.com/js/js_const.asp

Upvotes: 2

Amit Beckenstein
Amit Beckenstein

Reputation: 1332

You can't do so, at least not in run-time. I suggest you consider using TypeScript, which has the readonly modifier which will help you protect your variable from being overwritten during compile-time.

Upvotes: 0

Matheus Frez
Matheus Frez

Reputation: 324

Have two ways to create a const with default value, 1 object with properties mutable, 2 array, as you can see bellow.

const a = {};
cosnt b = [];

Upvotes: 3

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