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 "".

Answer 2

function parameterTest(bool) {    if(typeof bool !== 'undefined')    {      alert('the parameter exists...');    }    else    {      alert('The parameter doesn\'t exist...');    } }