CODEWITHSUNDEEP
javascriptfunctionecmascript-6
bigmugcup
bigmugcup

Reputation: 1351

Passing function as parameter and calling it javascript

I ran into an example of javascript code as below and am not clear on how it works. I am used to passing functions as callbacks, but I cannot seem to grasp how

  1. the variable func could take arguments (a, b), and
  2. why the function pair would be called in that fashion.

function cons(a, b) {
    const pair = func => {
        return func(a, b);
    };
    return pair;
}

function car(pair) {
    return pair((a, b) => {
        return a;
    });
}

function cdr(pair) {
    return pair((a, b) => {
        return b;
    });
}

console.log(cons(1, 2))
//  ƒ pair(func) {return func(a, b);}
console.log(car(cons(1, 2)))
// 1
console.log(cdr(cons(1, 2)))
// 2

Upvotes: 2

Views: 5234

Answers (2)

Ravindra Thorat
Ravindra Thorat

Reputation: 2002

in simple words, it's all happening because of closure! you are calling a function cons which internally returns another function definition which internally refers the properties a and b which are in the scope of cons [ a parent ] function.

to add more lights, let's do some simple changes and you can see it,

function cons(a, b) {
    const pair = func => {
        console.log('a: ', a);
        console.log('b: ', b);
    };
    return pair;
}

let childFun = cons(10, 30);
console.log(childFun());

now, cons returns some function which accepts one function as a parameter and executes that function on remembered a and b. in next, you are passing that closure function car function and car function is triggering that function by passing another callback which accepts both a and b but returns just b.

and in the same way, cdr function does with b

Upvotes: 1

Jack Bashford
Jack Bashford

Reputation: 44107

The variable func takes arguments a and b because they're parameters in the cons function. The function call occurs like that (I believe you're talking about a function being logged to the console) because cons returns a function, which you don't call - therefore, you pass more arguments (callback functions) like so:

function cons(a, b) {
    const pair = func => {
        return func(a, b);
    };
    return pair;
}

function car(pair) {
    return pair((a, b) => {
        return a;
    });
}

function cdr(pair) {
    return pair((a, b) => {
        return b;
    });
}

cons(1, 2)(console.log);
console.log(car(cons(1, 2)))
// 1
console.log(cdr(cons(1, 2)))
// 2

Upvotes: 2

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