ES6 TDZ(Temporary Dead Zone)

ES6-TDZ-explain.txt

The w in the w + 1 default value expression looks for w in the formal parameters' scope, 
but does not find it, so the outer scope's w is used. 
Next, The x in the x + 1 default value expression finds x in the formal parameters' scope, 
and luckily x has already been initialized, so the assignment to y works fine.

However, the z in z + 1 finds z as a not-yet-initialized-at-that-moment parameter variable, 
so it never tries to find the z from the outer scope.

As we mentioned in the "let Declarations" section earlier in this chapter, 
ES6 has a TDZ, which prevents a variable from being accessed in its uninitialized state. 
As such, the z + 1 default value expression throws a TDZ ReferenceError error.

tdz.js

var w = 1, z = 2;

function foo( x = w + 1, y = x + 1, z = z + 1 ) {
    console.log( x, y, z );
}

foo();                  // ReferenceError