JavaScript slice without parameters or brackets (code in CodeMirror sql-hint.js)

Question

I am adapting the sql-hint.js code from the excellent CodeMirror to my evil designs...

But I have encountered a line of JavaScript which is beyond me. I don't understand it, and cannot find anything that explains it.

It is line 34 in CodeMirror-master\addon\hint\sql-hint.js and it looks like this:

function getItem(list, item) {
     if (!list.slice) return list[item];     // <--- THIS LINE!!!!!
for (var i = list.length - 1; i >= 0; i--) if (getText(list[i]) == item)
    return list[i];
}

The command I do not understand is this:

if (!list.slice)

There are no parameters, and there are no brackets. What on earth is going on here? I assume magic...

Please note that it does not say this:

if (!list.slice())

Answer 1

JavaScript is a truthy/falsy language which means that we can assert whether or not a given object is true or false based on some criteria. In JavaScript the values that always report as false are: false, 0 (zero) "" (empty string), null, undefined, NaN.

The check here is looking to find out if list.slice is undefined and if so, is returning the list item at index item.

If you were to console.log(list.slice) you'd expect something like function () {}. We wouldn't want to call the function though since that would fail if slice was undefined.