javascript function expression within function

Question

The working javascript snippet below does not include validation as it is only being used for learning purposes. However, I am not understanding the flow of events after variable 'isBetween' is defined within the buildBoundDetector() function. Why does passing a number through variable 'f' work?

function buildBoundDetector( lowerBound, upperBound ) {
    var isBetween = function(number){       
        if(lowerBound <= number && number <= upperBound){
            return true;
        }
        return false;
    }
    return isBetween;
}

var f = buildBoundDetector( 1, 100 );
f(45);

return isBetween;, it returns the function object. So, f is the isBetween function and you are actually passing 45 to isBetween. — thefourtheye
– thefourtheye, Commented Mar 1, 2015 at 5:44
Time to learn about higher order functions: en.wikipedia.org/wiki/Higher-order_function — Felix Kling
– Felix Kling, Commented Mar 1, 2015 at 5:57

KSFT · Accepted Answer · 2015-03-01 05:45:22Z

2

buildBoundDetector() is a function that returns a function. In Javascript, you can assign a function to a variable. That's what buildBoundDetector() does. It defines an anonymous function, then assigns it to isBetween, then returns isBetween. f is set to the result of buildBoundDetector(), which is that function. Because f is a function, you can call it.

answered Mar 1, 2015 at 5:45

KSFT

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amahfouz · Accepted Answer · 2015-03-01 05:45:05Z

1

In JavaScript, and many other languages, functions can be treated as values. So your first function returns a value which itself is a reference to a function. Then, the returned function value is applied, like any other function, to the argument 45.

answered Mar 1, 2015 at 5:45

amahfouz

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