Question

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JavaScript: Is “z=(x||y)” same as “z=x?x:y” for non-boolean?

Are the following two lines of code equivalent in javascript?

a = b ? b : c

a = b || c

I want to express: “a should be assigned b if b is truthy, otherwise a should be assigned c”

I expect they would both work exactly the same, but I’m not 100% sure.

Answer 1

Yes. The two are almost exactly identical.

Both will first evaluate b. If it’s truthy, it’ll return b. Else, it’ll return c.

---

As pointed out by @thesystem, if you have a getter method on b, it’ll be called twice for the ternary, but only once for the or statement.

Test it using the following snippet:

var o = {};
o.__defineGetter__("b", function() {
    console.log('ran');
    return true;
});

var d = o.b || o.not;

console.log('-----');

var d = o.b ? o.b : o.not;

Here’s the fiddle: http://jsfiddle.net/bqsey/