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(\'t         


        

Answer 1

best way to check: if the param is not undefined

function parameterTest(param) {
    if (param !== undefined)
    ...

the param could be also a variable or a function name

Answer 2

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

Answer 3

null == undefined is true

if (arg == null){
    // arg was not passed.
}

Code example:

var button = document.querySelector("button");

function myFunction(arg){
  if(arg == null){
    alert("argument was not passed.");
  } else {
    alert("argument " + arg + " was passed.");
  }
}

<button onclick="myFunction('foo');">click to fire function w arg</button>
<br><br>
<button onclick="myFunction();">click to fire function w/o arg</button>

Answer 4

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 5

I know this is old, but this is my preferred way to check, and assign default values to functions:

function testParamFunction(param1, param2) {
    param1 = typeof param1 === 'undefined' ? null : param1;
    param2 = typeof param2 === 'undefined' ? 'default' : param2;

    // exit if the required parameter is not passed
    if (param1 === null) {
        console.error('Required parameter was not passed');
        return;
    }

    // param2 is not mandatory and is assigned a default value so 
    // things continue as long as param1 has a value
}

Answer 6

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