JavaScript River Example

river.js

// Water molecules can still be Numbers
// since they're immutable values in JS
1
2

// The story is not so good with arrays
// JS Arrays are mutable by default
// and so we start with an identity
[1, 2, 3]

// As before, lets give this a name
var yellow_river = [1, 2, 3];

// The identity can still have many names
var huang_he = yellow_river;
// var རྨ་ཆུ། = yellow_river; but tibetan doesn't work

// We're still missing our river value representation.

// Here's a snapshot function that extracts an
// immutable value from a mutable javascript array
function snapshot(array) {
    return Object.freeze(array.slice());
}

// Now we can extract the state at point in time from our river
snapshot(yellow_river);
// => [1, 2, 3]

// As before, a little trick to make our river flow
var timer = setInterval(function() {
    yellow_river.forEach(function(v, i) {
        yellow_river[i]++;
    });
}, 1000);

// And now as we look at the state of our river at different
// times, we see that the value has changed
snapshot(yellow_river);
// => [4, 5, 6]

// And we can still look via a different name
snapshot(huang_he);
// => [5, 6, 7]

// Now that we have all of the same tools, lets cross our river
var first_crossing = snapshot(yellow_river);

// And wait for a bit...
var d = Date.now(); while(Date.now() - d < 1200);

// Then cross it again
var second_crossing = snapshot(yellow_river);

// Now theres a slight problem - we made up our own
// representation of the river's value, so we have
// no way to compare the two crossing values.

// Let's write one:
function equal(a1, a2) {
    return Object.isFrozen(a1) &&
        Object.isFrozen(a2) &&
        a1.every(function(v, i) { return v == a2[i] });
}

// And as expected, our two values aren't the same
equal(first_crossing, second_crossing);
// => false