README.md

streamJS

Motivation: I wanted to write the following scala program in javascript. This uses streams for lazy evaluation.

def isPrime: Boolean = (start > 1) && (primes takeWhile { _ <= Math.sqrt(start) } forall { start % _ != 0 })

val primes = Stream.cons(2, Stream.from(3, 2) filter { _.isPrime })

So implemented a lazily evaluated Stream. The next element in the Stream is evaluated only when the take(n) method is called.

Methods

1. Constructor: Stream(head, step, filterFunction)

head (required) : First element of the stream
step (optional, defaults to 1) : Next element in stream (Arithmatic progression only)
filterFunction (optional, default is no filter) : A function that given an element of 
the stream returns true or false.

2. take (n)

Returns an array of the first n elements of the stream.
n : The number of elements to fetch.

3. cons(stream)

Creates and returns a new Stream which is generated by appending a stream to the current head.

4. filter(func)

Returns a new Stream with the filter set to the function func.

Usage

var s1 = new Stream(2,3);
console.log(s1.take(5));

[2,5,8,11,14]

var s2 = new Stream(99);
var s3 = s1.cons(s2);
console.log(s3.take(5));

[2,99,100,101,102]

function isFiver (n){
	if (n%5 ==0)
		return true;
	else
		return false;
}

var s4 = s1.filter(isFiver);
console.log(s4.take(5));

[5,20,35,50,65]

function isPrime(n){ 
	if (n == 2)
		return true;
	var x = Math.floor(Math.sqrt(n));
	var a = primes.take(x);
	return a.every ( function ( e ) { return n % e != 0 } );
}

var primes = new Stream(2).cons ( new Stream(3,2).filter(isPrime));

console.log(primes.take(5));

[2,3,5,7,11]