Represent pair data structure using functions.

Pair

Pair is a data structure which is used to combine two values.

(1, 4)
("sreedhar", "pavan")

Javascript does not provide this data structure. So let us implement this.

Represent pairs in JS

Pairs can be represented using an object or an array

//Represent pair using object
function pair(a,b){
    return {first: a, second: b};
}
function first(p){
    return p.first;
}
function second(p){
  return p.second;
}

//use the pair
const p = pair(12, 13)

const f = first(p)
console.log(f) // logs 12

const s = second(p)
console.log(s) // logs 13

//Represent pair using array.
function pair(a, b) {
  return [a, b];
}
function first(p) {
  return p[0];
}
function second(p) {
  return p[1];
}

//use the pair
const p = pair(12, 13);

const f = first(p);
console.log(f); // logs 12

const s = second(p);
console.log(s); // logs 13

But is there a way to represent pair in terms of functions without using object or array?

Think for some time before you proceed.
.
.
.
Yes there is a way

Represent pair using functions

Consider our pair function returns a function.

function pair(a, b) {
  return (value) => (value === 0 ? a : b);
}

Now this returned function when called with 0 returns the first element and when called with 1 returns the second element.

so we can implement first and second functions as

function first(p) {
  return p(0);
}

function second(p) {
  return p(1);
}

Finally with these functions we can represent pair using functions.

function pair(a, b) {
  return (value) => (value === 0 ? a : b);
}
function first(p) {
  return p(0);
}

function second(p) {
  return p(1);
}

//use the pair
const p = pair(12, 13);

const f = first(p);
console.log(f); // logs 12

const s = second(p);
console.log(s); // logs 13

Conclusion

I would never implement pair using functions. But it was fun to realise that I can.
I found this in a section in SICP book.