In JavaScript, how can I create a function with an optional parameter?

Question

Question: What is the proper way to define a function in JavaScript that takes optional parameters?

For example:

function myFunc(optionVar1) {     if(optionVar1 == undefined) {         ...     } else {         ...     } }  myFunc('10');  // valid function call myFunc();      // also a valid function call 

Is it proper to use a ? mark like Ruby in the function declaration like so to denote optional parameters:

function myFunc(optionVar1?) {...}  //  <--- notice the ? mark 

Answer 1

There is no syntax in Javascript that specifies that a parameter is optional (or required). All parameters are optional. If they aren't specified they're undefined so you need to check for that. For example, this function will in effect create a default value of 10 for the parameter:

function myfunc(someParam) {   if (someParam === undefined) {     someParam = 10;   }   ... } 

Also you can access the parameters programmatically using the arguments property.

Lastly, if you have more than about 3-4 parameters it's generally advisable to use an anonymous object instead.

Answer 2

Actually, all parameters are optional in JS functions. There is no warning or error if you omit a parameter.

You can set defaults like

function throw_cat(dist){   dist = typeof dist=='undefined' ? 20 : dist;    //OR   dist = dist || 20; //this will assign it to 20 if you pass 0 or another 'falsy' value, though. May be good if you expect a string. String '0' stays, '' or null assigns the default   //etc...   }