Question

Can someone please explain the steps this program takes and the order in which they are taken in order to produce the result “false”

function negate(func) {
  return function(x) {
    return !func(x);
  };
}
var isNotNaN = negate(isNaN);
show(isNotNaN(NaN));

Answer 1

  // 1. A function called "negate is declared
function negate(func) {  // 3. The "isNaN" function is received by negate().

  return function(x) {   // 4. A function is returned from the negate() call
    return !func(x);     //     that invokes the "isNaN" function, and returns
  };                     //     the logical opposite of its return value.

}

                  // 2. The built in "isNaN" function is passed to negate()
var isNotNaN = negate(isNaN);
 // 5. The function returned from negate() is assigned to the "isNotNaN" variable

         // 6. The "isNotNaN" function is invoked, and passed the "NaN" value.
show(isNotNaN(NaN));
 // 7. The result returned from "isNotNaN" is passed to the show() function.

The end result is that you have a function that returns the opposite of the isNaN function.

Seems like overkill when you can just call isNaN with ! yourself.

show( !isNaN(NaN) );  // gives the same result