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Today (in 2014) is it advisable to use the const keyword to declare constants in JavaScript? Are there still browser compatibility issues? Also, is there a way to declare a global constant?

Note: This questions is not a duplicate, I am asking for the year 2014. Other posts in Stack Overflow date back to 2008-2012

FranXh
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  • Yes, there are still compatibility issues with older browsers. Please specify the targetted browser/engine versions if you need more detailed information. – Bergi Jun 23 '14 at 16:07
  • The `const` keyword, introduced with JavaScript 1.7 and still apparent in current Firefox, does behave different from the `const` keyword that is envisioned by EcmaScript harmony. – Bergi Jun 23 '14 at 16:09
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    There are only compatibility issues if there's a browser you support that isn't compatible. Only you can know if it's compatible for your purposes. – cookie monster Jun 23 '14 at 16:12
  • ES6 `const` browser compatibility: http://kangax.github.io/compat-table/es6/#const – Ian Jun 23 '14 at 16:17
  • Whoever voted to close for "primarily opinion based" ... browser compatibility is not an opinion. – Sterling Archer Jun 24 '14 at 02:52
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    I would strongly advise against it; you should be writing JavaScript in strict mode, which explicitly disallows `const`. If it works, odds are you’re doing something wrong. ES6 won’t be safe for widespread use for a while. – Ry- Jun 24 '14 at 04:31

2 Answers2

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As at June 2014, MDN says this:

The current implementation of const is a Mozilla-specific extension and is not part of ECMAScript 5. It is supported in Firefox & Chrome (V8). As of Safari 5.1.7 and Opera 12.00, if you define a variable with const in these browsers, you can still change its value later. It is not supported in Internet Explorer 6-10, but is included in Internet Explorer 11. The const keyword currently declares the constant in the function scope (like variables declared with var).

Firefox, at least since version 13, throws a TypeError if you redeclare a constant. None of the major browsers produce any notices or errors if you assign another value to a constant. The return value of such an operation is that of the new value assigned, but the reassignment is unsuccessful only in Firefox and Chrome (at least since version 20).

const is going to be defined by ECMAScript 6, but with different semantics. Similar to variables declared with the let statement, constants declared with const will be block-scoped.

Reference here

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const was introduced in JavaScript 1.5, was a Mozilla-specific extension and not part of ECMAScript 5.

However, the ES6 draft introduces a different const. Similar to variables declared with the let statement, constants declared with const will be block-scoped.

So basically:

  • Some browsers implement the old non standard way.
  • Other browsers allow variables declared with const to change its value later.
  • Other browsers don't implement it at all.
  • Currently no browser (AFAIK) implements the ES6 way.

See MDN article.

Oriol
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    Which I think leads to the conclusion that unless you're writing code that is only going to execute against one specific JS engine (like running in node.js), you pretty much can't use `const` now because the behavior is variable and going to change in the future. – jfriend00 Jun 23 '14 at 16:15