javascript - Can you add a condition to a variable declaration?

Question

This doesn't make sense to me, but I have a feeling I saw a code using this:

var abc = def || ghi;

My question is, is this valid? Can we add a condition to a variable declaration? I imagine the answer is no but I have this at the back of my mind that I saw something similar in code once.

Answer 1

This gives to abc the value of def if it isn't falsy (i.e. not false, null, undefined, 0 or an empty string), or the value of ghi if not.

This is equivalent to:

var abc;
if (def) abc = def;
else abc = ghi;

This is commonly used for options:

function myfunc (opts) {
    var mything = opts.mything || "aaa";
}

If you call myfunc({mything:"bbb"}) it uses the value you give. It uses "aaa" if you provide nothing.

In this case, in order to let the caller wholly skip the parameter, we could also have started the function with

opts = opts || {};