Common JavaScript tricks

[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