Lazy evaluation in javascript, with a simple stream implementation because we can

lazy.js

// Lazy evaluation in javascript
// call by name: lazily evaluate the arguments passed into the function
// call by value: eagerly evaluate the arguments passed into the function
// call by need: memoized version of call by name
// memoization: avoid recomputation for the same inputs by caching the results
//
// Javascript by default uses call by value, however by using Object.defineProperty(),
// lazy evaluation in javascript object properties can be performed
//
// e.g. an infinite data structure - the stream, can only exist with lazy evaluation (else infinite recursion)

function Stream(value) {
        this.value = value

        Object.defineProperty(this, 'next', {
                get: () => new Stream(value + 1)
        })
}

Stream.prototype.takeUntil = function (n, acc) {
        const _acc = acc || []

        if (n < this.value) { return }

        if (n === this.value) { return _acc }

        _acc.push(this.value)
        return this.next.takeUntil(n, _acc)
}

function main() {
        const stream = new Stream(10)
        console.log(stream.takeUntil(20))
}

main()