Optional Arguments

All function arguments are optional and when omitted are given the value "undefined".

function methodOne( a, b?) { 
} 

Default Values

Added in ES 2015.
Instead of always checking for undefined values the default values can be placed in the function declaration.

function methodOne( a = 10, b = 20) { 
   console.log(a + b); 
} 

You can even use a function to retreive a default value
The getDefaultValue will only be called when the second argument is omitted.

function getDefaultValue() { 
   return 20; 
} 

function methodTwo( a = 10, b = getDefaultValue() ) { 
   console.log(a + b); 
} 

You can even perform operations in the function declaration
The default values are evaluated at call time.

function methodThree( a , b = a + a , c = b * b ) { 
   console.log(c); 
}