More Goodies

One of the useful shortcuts is the Stream.range(min, max) function. It returns a stream with the natural numbers ranging from min to max inclusive.

var s = Stream.range(10, 20);
s.print(); // prints the numbers from 10 to 20

You can use map, filter, and walk on your streams.

s.map(f) takes an argument f, a function, and runs f on every element of the stream; it returns the stream of the return values of that function. So you can use it to, for example, double the numbers in your stream:

function doubleNumber(x) {
  return 2 * x;
}

var numbers = Stream.range(10, 15);
numbers.print(); // prints 10, 11, 12, 13, 14, 15
var doubles = numbers.map(doubleNumber);
doubles.print(); // prints 20, 22, 24, 26, 28, 30

Cool, right? Similarly s.filter(f) takes an argument f, a function, and runs f on every element of the stream; it then returns the stream containing only the elements for which f returned true. So you can use it to only keep certain elements in your stream. Let's construct a stream keeping only the odd numbers of an original stream using this idea:

function checkIfOdd(x) {
  if (x % 2 == 0) {
    // even number
    return false;
  } else {
    // odd number
    return true;
  }
}
var numbers = Stream.range(10, 15);
numbers.print(); // => 10, 11, 12, 13, 14, 15
var onlyOdds = numbers.filter(checkIfOdd);
onlyOdds.print(); // => 11, 13, 15

Useful, yes? Finally s.walk(f) takes an argument f, a function, and runs f on every element of the stream, but it doesn't affect the stream in any way. Here's another way to print the elements of stream:

function printItem(x) {
  console.log( 'The element is: ' + x );
}
var numbers = Stream.range(10, 12);

numbers.walk(printItem);
// => The element is: 10
// => The element is: 11
// => The element is: 12

One more useful function: s.take(n) returns a stream with the first n elements of your original stream. That's useful for slicing streams:

var numbers = Stream.range(10, 100); // numbers 10...100
var fewerNumbers = numbers.take(10); // numbers 10...19
fewerNumbers.print();

A few other useful things: s.scale(factor) multiplies every element of your stream by factor; and s.add(t) adds each element of the stream s to each element of the stream t and returns the result. Let's see an example of this:

var numbers = Stream.range(1, 3);
var multiplesOfTen = numbers.scale(10);
multiplesOfTen.print(); // prints 10, 20, 30
numbers.add(multiplesOfTen).print(); // prints 11, 22, 33

Although we've only seen streams of numbers until now, you can also have streams of anything: strings, booleans, functions, objects; even arrays or other streams. Please note however that your streams may not contain the special value undefined as an item.

Continue to the next section and start playing with infinity.