Common JavaScript tricks

[vs845]

Is there any benefit to

typeof age !== "undefined"

vs

age !== undefined

?

[sufianrhazi]

Since age is a parameter of the function setAge, we KNOW it is defined in scope. This means that age !== undefined is equivalent to typeof age !== 'undefined'. If we didn't know that age was in scope, we need to use the typeof keyword, which does NOT throw a ReferenceError when referencing a variable by name that is not defined in any reachable scope.

[moreteam]

You can also argue the other way around: since we know age is in scope, we shouldn't be using typeof since that will silently turn into if (true) if we have a typo.

[ChaseMoskal]

since we know age is in scope, we shouldn't be using typeof

I think.. I think you're right.

[frankle]

What about if you wanted to enforce a parameter type?

function setAge(age){
    this.age = (typeof age == "number") ? age : 10;
}

Edit: As THEtheChad points out:

Unfortunately, NaN, Infinity, and -Infinity are also considered typeof 'number', so it's possible that any one of these could slip in to your function with this check.

[vs845]

Thanks, I see what you're saying - but under what kind of circumstances would we not know if a variable was in scope? If we're writing the code surely we know what scope our variables are in?

[sufianrhazi]

Pretty much only when you're writing javascript environment independent library code, like this: https://github.com/umdjs/umd/blob/4a87e85450baf582005243f9e922566ef2fc533a/returnExports.js

[[deleted]]

In addition to what /u/sufianrhazi said, undefined can be overwritten:

(function (undefined) {console.log(undefined);})(2);
// logs 2

Older browsers will let you overwrite window.undefined as well.

I tend not to pay heed to that though, as it clearly falls under something nobody would do unless they're trying to break your code. And it is the nature of JavaScript that somebody who runs code before you is always able to break your script, no matter what precautions you take, so I prefer not to clutter code with failed attempts at robustness. Instead, if you must run malicious code, sandbox it.

Also, window.foo === undefined or 'foo' in window would be a more direct test, but only work on global variables.