How do you implement a guard clause in JavaScript?

Question

I want to guard my functions against null-ish values and only continue if there is "defined" value.

After looking around the solutions suggested to double equal to undefined: if (something == undefined). The problem with this solution is that you can declare an undefined variable.

So my current solution is to check for null if(something == null) which implicetly checks for undefined. And if I want to catch addionalty falsy values I check if(something).

See tests here: http://jsfiddle.net/AV47T/2/

Now am I missing something here?

Matthias

Answer 1

The standard JS guard is:

if (!x) {
    // throw error
}

!x will catch any undefined, null, false, 0, or empty string.

If you want to check if a value is valid, then you can do this:

if (Boolean(x)) {
    // great success
}

In this piece, the block is executed if x is anything but undefined, null, false, 0, or empty string.

-tjw

Answer 2

The only safe way that I know of to guard against really undefined variables (meaning having variable name that were never defined anywhere) is check the typeof:

if (typeof _someUndefinedVarName == "undefined") {
   alert("undefined");
   return;
}

Anything else (including if (!_someUndefinedVarName)) will fail.

Basic example: http://jsfiddle.net/yahavbr/Cg23P/

Remove the first block and you'll get:

_someUndefinedVarName is not defined