What's this JavaScript syntax?

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;
}

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.