Question

As a new javascript developer, I have spent some time with this code snippit from Chapter 6 of Eloquent Javascript, I am still trying to understand the following code example :

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

Where it particular loses me is the following line, I just don’t understand the call in general and where the variable/value for NaN comes from:

document.writeln(isNotNaN(NaN));

Answer 1

I think xdazz pretty much covered it, but since you said you still don’t get it maybe it would help to hear the explanation in somebody else’s words.

This line:

var isNotNaN = negate(isNaN);

…declares a variable isNotNan that is assigned equal to the result of a call to the negate() function, passing isNan as a parameter.

That parameter isNan is actually a function as described by MDN, but negate() would accept any function as a parameter, you could say for example var isNotFinite = negate(isFinite);.

Now the negate() function actually creates and returns another function, so after that line runs isNotNan references that returned function, which means you can call it as isNotNan(someVal).

So then the line:

document.writeln(isNotNaN(NaN));

… calls isNotNan() and passes it NaN as a parameter, and the result is written out to the document.

“I just don’t understand…where the variable/value for NaN comes from”

NaN is a property of the global object. To oversimplify, it is a constant provided to you by the JS environment.

Regarding how the negate() function works, it relies on the concept of “closures”, which means that functions declared inside negate() have access to its variables and parameters even after negate() completes. You’ll notice that the returned function references the func parameter. So when you call the returned function via isNotNaN() it can still access that original func parameter that is set to the isNan function.

The effect is kind of like doing this:

var isNotNaN = function(x) {
    return !isNan(x);
};