default parameters

Allow formal parameters to be initialized with default values if no value or undefined is passed.

Syntax

function f(a, b = 37) {
  // when called, if no value or undefined is passed as second argument, b will have 37 as value.
}

Examples

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

multiply(5); // 5

Before default parameters,  if no value is provided for b in the call, its value would be undefined  when evaluating a*b and the call to multiple would have returned NaN;. So the general strategy for setting defaults is to test parameter values in the body of the function and assign a value if they're undefined:

b = typeof b == 'undefined'?  b : 1;

Here's another example:

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' 

For the second case, even if the second argument is set explicitly to undefined when calling, the value of the color argument is the default one.

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

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

The default argument gets evaluated at call time, so unlike e.g. in Python, a new Object is created each time the function is called.
This even applies to functions and variables:

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

callSomething(); //throws a ReferenceError

let (something = function() "sth") {
  callSomething(); //"sth"
}

Browser compatibility

Specification

• part of the draft ECMAScript 6 specification
• see the original proposal at ecmascript.org

See also