Question

I have just come across a function in JavaScript which has return !1.

What does this actually mean? Why would you return !1 or return !0?

Here is the function that I came across:

function convertStringToBoolean(a) {
    typeof a == "string" && (a = a.toLowerCase());
    switch (a) {
    case "1":
    case "true":
    case "yes":
    case "y":
    case 1:
    case !0:
        return !0;
    default:
        return !1
    }
}

Answer 1

In immediate response to your question:

• return !1 is equivalent to return false
• return !0 is equivalent to return true

In the specification – 11.4.9 Logical NOT Operator – it states that when you place an exclamation mark ! in front, the result is evaluated as Boolean and the opposite is returned.

Example:

var a = 1, b = 0;
var c = a || b;
alert("c = " + c + " " + typeof c); // here typeof c will be "number"

a = !0, b = !1;
c = a || b;
alert("c = " + c + " " + typeof c); // here typeof c will be "boolean"

I mostly see this in a code passed through Google’s JS optimiser. I think it is mostly done to achieve shortness of the code.

It is often used when a strictly Boolean result is needed – you may see something like !!(expression). Search in jQuery, for example.