Can you use constant variables in JavaScript? [duplicate]

This question already has answers here: Are there constants in JavaScript? (33 answ开发者_运维知识库ers) Closed 5 years ago.

I read on one site that you can make constant variables in JavaScript like:

const x = 20;

but on another site I read that you can't. So I am confused now what is it now?

Also in Visual Studio 2010 when I write const it underlines it in the JavaScript file and shows syntax error.

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const is a proposed feature of ECMAScript Harmony (together with a properly block-scoped let it is supposed to replace var and implicit globals). ECMAScript Harmony is a grab-bag of ideas for the next versions of ECMAScript.

const was also a part of ECMAScript 4.

ECMAScript 4 was never released and never will be, and ECMAScript Harmony will only be released in a couple of years. Therefore, you cannot reliably use it.

There are some implementations or derivatives of ECMAScript that implement const (ActionScript, for example). There are also some implementations that accept const as a synonym for var (IOW, you can use const, but it won't give you any protection.)

However, unless you absolutely can guarantee that your code will only run on very specific versions of very specific implementations of very specific derivatives of ECMAScript, it's probably better to avoid it. (Which is a real shame, because const and especially let are a huge improvement over var and implicit globals.)

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if you're looking for a read-only variable, you simulate that with something like

var constants = new (function() {
    var x = 20;
    this.getX = function() { return x; };
})();

and then use it like

constants.getX()

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The solution is to create an object and put all your constants in the object:

const={};

const.x=20;

const.y=30;

Object.freeze(const);  // finally freeze the object

Usage:

var z=const.x + const.y;

Any attempt to modify the variable will generate an error:

const.x=100;  <== raises error

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There's no const in ECMAScript (disregarding the dead 4.0 version, and ActionScript).

The const is available in JScript.NET, and some recent versions of JS engines e.g. Firefox, Opera 9, Safari as a vendor-specific extension.

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JavaScript ES6 (re-)introduced the const keyword which is supported in all major browsers.

Variables declared via const cannot be re-declared or re-assigned.

Apart from that, const behaves similar to let.

It behaves as expected for primitive datatypes (Boolean, Null, Undefined, Number, String, Symbol):

const x = 1;
x = 2;
console.log(x); // 1 ...as expected, re-assigning fails

Attention: Be aware of the pitfalls regarding objects:

const o = {x: 1};
o = {x: 2};
console.log(o); // {x: 1} ...as expected, re-assigning fails

o.x = 2;
console.log(o); // {x: 2} !!! const does not make objects immutable!

const a = [];
a = [1];
console.log(a); // 1 ...as expected, re-assigning fails

a.push(1);
console.log(a); // [1] !!! const does not make objects immutable

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I believe there is something which is not with var , is a global variable....there is no const in js.

var a="test" is different from a="test"

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No, there is no data type "const" in JavaScript.

JavaScript is a loosely typed language. Every kind of variable is declared with a var