JavaScript Functions – Function Declaration as Var vs Function

Doug Crockford discusses let at this point in his talk, "The Better Parts".

The point is, let avoids a source of misunderstanding, esp. for programmers with expectations set by languages with block-scope. A var has function scope (it declares a variable that's visible throughout the function) even though it looks like it has block scope.

var might possibly still be useful in an extreme case like machine-generated code, but I'm stretching hard there.

(const is also new and has block scope. After let x = {'hi': 'SE'} you can reassign to x, while after const y = x you cannot reassign to y. That's often preferrable since it keeps something from accidentally changing out from under you. But to be clear, you can still modify the object y.hi = 'SO' unless you freeze it.)

Realistically, your impression is right on for ES6: Adopt let and const. Stop using var.

(In another performance of "The Better Parts", Doug says why === was added rather than fixing the problems of ==. == produces some "surprising" results, so just adopt ===.)

A Revealing Example

Mozilla Developer Network gives an example where var does not work as intended. Their example is a realistic one that sets onclick handlers in a web page. Here's a smaller test case:

var a = [];
(function () {
   'use strict';
   for (let i = 0; i < 5; ++i) { // *** `let` works as expected ***
     a.push( function() {return i;} );
   }
} ());
console.log(a.map( function(f) {return f();} ));
// prints [0, 1, 2, 3, 4]

// Start over, but change `let` to `var`.
// prints [5, 5, 5, 5, 5]

var trips us up because all loop iterations share the same function-scoped i variable, which has the value 5 after the loop finishes.

Another Telling Example

function f(x) {
    let y = 1;
    if (x > 0) {
        let y = 2;  // `let` declares a variable in this block
    }
    return y;
}
[f(1), f(-1)]  // --> [1, 1]

// Start over, but change `let` to `var`.
// --> [2, 1]

let declares block-scoped variables. var confuses us by referring to the same variable throughout the function.

Is a function getting a value from another function considered pure?

That depends on what the other function does, and what the calling function does. Impurity is infectious, purity isn't.

Calling a pure function doesn't change the purity of the calling function. Calling an impure function automatically makes the calling function impure also.

So, in your example, it depends on the purity of the part you left out: if that is pure, then the whole function is pure.

If this is a pure function, what is the difference from just using a global DEFAULT_SEPARATOR constant instead?

Nothing. A function that always returns the same value is indistinguishable from a constant. In fact, that is exactly how constants are modeled in λ-calculus.