In JavaScript, there is no defined way to assign default values to function parameters that are not passed. So programmers usually check for parameters with the undefined value (as it is the default value for missing parameters) and assign the default values to them. The following example demonstrates 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);

This can be done in an easier way by providing a default value to function arguments. Here is the code that demonstrates how to do this:

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

In the preceding code block, since we've passed first two arguments in the function calling statement, the default values (that is x = 1 and y = 2) will be overwritten with our passed values (that is x = 6 and y = 7). The third argument is not passed, hence its default value (that is z =3) is used.

Also, passing undefined is considered as missing an argument. The following example demonstrates this:

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

A similar thing happens here. If you want to omit the first argument, just pass undefined in that.

Defaults can also be expressions. The following example demonstrates this:

 function myFunction(x = 1, y = 2, z = x + y) {
   console.log(x, y, z); // Output "6 7 13"
 }
 myFunction(6,7);

Here, we're making use of the argument variables themselves inside a default argument value! That is, whatever you pass as the first two arguments, if the third argument is not passed it'll take the value of the sum of the first two arguments. Since we passed 6 and 7 to the first and second argument, z becomes 6 + 7 = 13.