Are there constants in JavaScript?

Question

Is there a way to use constants in JavaScript?

If not, what\'s the common practice for specifying variables that are used as constants?

Answer 1

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

If you really need an immutable and absolutely constant object: Just use const ALL_CAPS to make your intention clear. It is a good convention to follow for all const declarations anyway, so just rely on it.