javascript - How to test if a parameter is provided to a function?

Question

I was wondering, can you create a function with an optional parameter.

Example:

function parameterTest(test)
{
   if exists(test)
   {
     alert('the parameter exists...');
   }
   else
   {
     alert('The parameter doesn't exist...');
   }
}

So if you call parameterTest() then the result would be a message "The parameter doesn't exist...". And if you call parameterTest(true) then it would return "the parameter exists...".

Is this possible?

Answer 1

This is a very frequent pattern.

You can test it using

function parameterTest(bool) {
  if (bool !== undefined) {

You can then call your function with one of those forms :

 parameterTest();
 parameterTest(someValue);

Be careful not to make the frequent error of testing

if (!bool) {

Because you wouldn't be able to differentiate an unprovided value from false, 0 or "".