JavaScript Variable fallback

Question

Please can someone explain to me what this line of code does:

var list  = calls[ev] || (calls[ev] = {});

My best guess:

It\'s setting t

Answer 1

Your guess is right. This is a common way to declare "default" values for variables in JavaScript.

function foo(bar) {
    var bar = bar || 0; //This sets bar to 0 if it's not already set
    console.log(bar);
}

The way this works is that in JavaScript, an undefined variable is falsy, meaning that in any boolean comparaison operation, it will evaluate to false. You can then use the OR operator to combine two values and it will return the first value that evaluates to true.