Javascript Nested Function Evaluation

Question

Hi I am confused with this Javascript function:

var currying = function(a) {
    return function(b){
        return function(c){
            return function(d){
                return a + b /c * d;
                };
            };
        };
    };
var a = currying(4)(8);
var b = a(2)(6);
console.log(b);

It outputs 28 but I am not sure how did Javascript evaluated the function. I also have learned that a = 4, b = 8, c = 2 and lastly d = 6. Thank you for somebody that will be able to explain the function.

synthet1c · Accepted Answer · 2016-12-03 13:22:48Z

When you use currying you are writing a function that takes one argument, that will return a function taking the next argument until all arguments are supplied.

In this case you need to call the returned function 4 times until you get to the last function and evaluate all the arguments.

This is good because you can pass in arguments at different times during the execution of your code and you can create new functions with arguments already set within the closure of the final functions. eg.

const fourPlusTwo = currying(4)(2)

now you can use the new function fourPlusTwo anywhere in your code and you will have those arguments baked in the closure of the remaining two functions.

The code you have is a non standard example but maybe if you needed to calculate tax throughout your app you could do something like.

const inclusiveTax = rate => amount => {
  return '$' + (amount * (rate / (100 + rate))).toFixed(2)
}

const norwayIncomeTax = inclusiveTax(60.2)
const strayaIncomeTax = inclusiveTax(32.5)
const muricaIncomeTax = inclusiveTax(31.5)

console.log(
  norwayIncomeTax(50000),
  strayaIncomeTax(50000),
  muricaIncomeTax(50000)
)

Using just one function you have curried the tax rate for 3 separate countries and returned functions waiting for the amount.

Xinzoop · Accepted Answer · 2016-12-03 13:23:27Z

2

You should know the difference between function object and function call. like: var a = function(v){return v + 1;} a is a function object. Then a(2) invokes function a.

Try to understand the procedure step by step.

currying is a function which return another function.

so currying(4) returns a function(a is given value 4):

function(b){
    return function(c){
        return function(d){
            return 4 + b /c * d;
            };
        };
    };
};

then currying(4)(8) also is 'var a' returns another function:

    function(c){
        return function(d){
            return 4 + 8 /c * d;
            };
        };
    };

invoking a(2) returns a function object:

        function(d){
            return 4 + 8 / 2 * d;
            };
        };

finally a(2)(6) returns 4 + 8 / 2 * 6 which is 28.

answered Dec 3, 2016 at 13:23

Xinzoop

1114 bronze badges

1 Comment

Cham Over a year ago

If I do it this way var a = currying(4)(8)(2)(6) will it result the same?

Maciej Kwas · Accepted Answer · 2016-12-03 12:25:08Z

1

This is kind of lexical scoping called Closures

Basically: A closure is a function within function that has access to all parent's variables and parameters. As every parameter in javascript is by default passed by reference, it can even modify parent's variables so you can later execute parent function to see the changes. The great example is jquery's ready function that wraps all other functions within.

You can read more about this here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Closures

answered Dec 3, 2016 at 12:25

Maciej Kwas

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Comments

codemonkey65 · Accepted Answer · 2016-12-03 12:27:05Z

This is a very convoluted example of currying - hence the name of the main function.

Currying originates in the field of functional programming and to fully understand it I would suggest that you do some reading, especially as it is implemented in javascript.

To give you some pointers:

In the line:

var a = currying(4)(8);

the first function is called with a parameter of 4; the result of that function call is another function which is then called immediately with a parameter of 8.

Ultimately, all that happens is that the line:

return a + b / c * d;

is executed with the values 4, 8, 2 and 6 for each of the respective variables.

Normal arithmetic rules are applied to give you an answer of 28 (divide first, then multiply and finally add).

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