function [name]([param1[ = defaultValue1 ][, ..., paramN[ = defaultValueN ]]]) 
{
 statements
}

function multiply(a, b) {
var b = (typeof b !== 'undefined') ? b : 1;

return a*b;
}

multiply(5); // 5

function multiply(a, b = 1) {
 return a*b;
}

multiply(5); // 5

function setBackgroundColor(element, color = 'rosybrown') {
 element.style.backgroundColor = color;
}

setBackgroundColor(someDiv);   // color set to 'rosybrown'
setBackgroundColor(someDiv, undefined); // color set to 'rosybrown' too
setBackgroundColor(someDiv, 'blue'); // color set to 'blue'

function append(value, array = []) {
array.push(value);
return array;
}

append(1); //[1]
append(2); //[2], not [1, 2]

function callSomething(thing = something()) { return thing }

function something(){
 return "sth";
}

callSomething(); //sth

function singularAutoPlural(singular, plural = singular+"s",
       rallyingCry = plural + " ATTACK!!!") {
 return [singular, plural, rallyingCry ];
}

//["Gecko","Geckos", "Geckos ATTACK!!!"]
singularAutoPlural("Gecko");

//["Fox","Foxes", "Foxes ATTACK!!!"]
singularAutoPlural("Fox","Foxes");

//["Deer", "Deer", "Deer ... change."]
singularAutoPlural("Deer", "Deer", "Deer peaceably and respectfully
 petition the government for positive change.")