What's this JavaScript syntax?

Question

I am new to JavaScript. The following code is from some production codebase. The regDefinition is passed in JSON form. But I am not quite sure about the syntax in the method body. Especially the || and [] parts.

function getCookieValue(regDefinition) {
    return (document.cookie.match(regDefiniation.regEx) || [])[regDefiniation.index] || null;
}

Answer 1

It looks like someone has made a lot of effort to make this very hard to read.

If I interpret it right, it does something like this:

• call the match method.
• it returns an array of matches, or nothing (null, undefined?). If it doesn't return anything, default to an empty array.
• Of the array, get the element with index 'regDefiniation.index'.
• If that item doesn't exist (which can be the case for matches, and will always be the case for the empty default array), return null.

Answer 2

There are some good answers here, but nobody seems to really be explaining why you'd do

(foo || [])[bar]; // or similarly (foo || {})[bar]

instead of just

foo[bar]

Consider case the RegExp failed,

var foo = null, bar = 0;

Now without anything special you'd get an error thrown and the code would stop

foo[bar]; // TypeError: Cannot read property '0' of null

However the parenthesised-or version will be different

(foo || [])[bar]; // undefined (note, no error)

This is because the result of (null || []) is [], and you can now try to read a property of it safely