Default parameter values for functions

With ES2015, it is also possible to define default parameter values for functions. The following is an example:

function sum(x = 1, y = 2, z = 3) { 
  return x + y + z; 
} 
console.log(sum(4, 2)); // outputs 9 

As we are not passing z as a parameter, it will have a value of 3 by default. So, 4 + 2 + 3 == 9.

Before ES2015, we would have to write the preceding function as in the following code:

function sum(x, y, z) { 
  if (x === undefined) x = 1; 
  if (y === undefined) y = 2; 
  if (z === undefined) z = 3; 
  return x + y + z; 
} 

Or, we could also write the code as follows:

function sum() { 
  var x = arguments.length > 0 && arguments[0] !== undefined ? arguments[0] : 1; 
  var y = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : 2; 
  var z = arguments.length > 2 && arguments[2] !== undefined ? arguments[2] : 3; 
  return x + y + z; 
} 

With ES2015, we can save a few lines of code using the default parameter values functionality.