In JavaScript there is no defined way to assign the default values to the function parameters that are not passed. So, the programmers usually check for the parameters with the undefined value (as it is the default value for the missing parameters), and assign the default values to them. Here is an example, which shows how to do this:

function myFunction(x, y, z)
{
  x = x === undefined ? 1 : x;
  y = y === undefined ? 2 : y;
  z = z === undefined ? 3 : z;

  console.log(x, y, z); //Output "6 7 3"
}
myFunction(6, 7);

ES6 provides a new syntax that can be used to do this in an easier way. Here is the code which demonstrates how to do this in ES6:

function myFunction(x = 1, y = 2, z = 3)
{
  console.log(x, y, z); // Output "6 7 3"
}

myFunction(6,7);

Also, passing undefined is considered as missing an argument. Here is an example to demonstrate this:

function myFunction(x = 1, y = 2, z = 3)
{
  console.log(x, y, z); // Output "1 7 9"
}

myFunction(undefined,7,9);

Defaults can also be expressions. Here is an example to demonstrate this:

function myFunction(x = 1, y = 2, z = 3 + 5)
{
  console.log(x, y, z); // Output "6 7 8"
}

myFunction(6,7);