Is there a better way to do optional function parameters in JavaScript? [duplicate]

Answer 1

The test for undefined is unnecessary and isn't as robust as it could be because, as user568458 pointed out, the solution provided fails if null or false is passed. Users of your API might think false or null would force the method to avoid that parameter.

function PaulDixonSolution(required, optionalArg){
   optionalArg = (typeof optionalArg === "undefined") ? "defaultValue" : optionalArg;
   console.log(optionalArg);
};
PaulDixonSolution("required");
PaulDixonSolution("required", "provided");
PaulDixonSolution("required", null);
PaulDixonSolution("required", false);

The result is:

defaultValue
provided
null
false

Those last two are potentially bad. Instead try:

function bulletproof(required, optionalArg){
   optionalArg = optionalArg ? optionalArg : "defaultValue";;
   console.log(optionalArg);
};
bulletproof("required");
bulletproof("required", "provided");
bulletproof("required", null);
bulletproof("required", false);

Which results in:

defaultValue
provided
defaultValue
defaultValue

The only scenario where this isn't optimal is when you actually have optional parameters that are meant to be booleans or intentional null.