Conditionally initializing a constant in Javascript

ES6 onwards we have const.

This is not allowed:

const x; //declare first
//and then initialize it
if(condition) x = 5;
else x = 10;

This makes sense because it prevents us from using the constant before it's initialized.

But if I do

if(condition)
    const x = 5;

else 
    const x = 10;

x becomes block scoped.

So how to conditionally create a constant?

Your problem, as you know, is that a const has to be intialised in the same expression that it was declared in.

This doesn't mean that the value you assign to your constant has to be a literal value. It could be any valid expression really - ternary:

const x = IsSomeValueTrue() ? 1 : 2;

Or maybe just assign it to the value of a variable?

let y = 1;
if(IsSomeValueTrue()) {
    y = 2;
}

const x = y;

You could of course assign it to the return value of a function, too:

function getConstantValue() {
	return 3;
}

const x = getConstantValue();

So there's plenty of ways to make the value dynamic, you just have to make sure it's only assigned in one place.

If ternary operator isn't an option for its unreadability, the only other option is IIFE, which is cumbersome but can be read fluently:

const x = (() => {
  if (condition)
    return 5
  else
    return 10
})();

The semantics of const is that it is assigned once. It should be let for this use case:

let x;
if(condition) x = 5;
else x = 10;

From my personal experience, ~95% of variables are const. If a variable has to be be let, just let it be itself; the probability of bugs caused by accidental reassignments is negligible.

Assuming that the const is going to be declared in both instances, you could use a ternary assignment:

const x = condition ? 5 : 10;

I suggest this solution with a Singleton Pattern implementation:

var Singleton = (function () {
    var instance;

    function createInstance() {
		// all your logic here
		// so based on your example:
		// if(condition) return 5;
		// else return 10;
    }

    return {
        getInstance: function () {
            if (!instance) {
                instance = createInstance();
            }
            return instance;
        }
    };
})();

const x = Singleton.getInstance();