X.js | Associative Variables in JavaScript

X.js is an open source library for associative variables in JavaScript.

X.js is a library to emulate associative variables in JavaScript. It provides a constructive API to create XVAR objects wrapping a value property whose runtime value updates synchronously after its parents. The library uses a combination of intuitive syntax, parent-children update trees, modified getters and setters, and safe defaults, to provide an interface with associative feel.

You can read more about the origins of the project in this blog post,

Download the latest stable version from the distribution folder or fork the project on github.

Oh, and in case you were wondering… X.js is read “big x dot js” ;)

Why Associative Variables?

Some time ago I realized that, when teaching newcomers how to code, they often had a hard time wrapping their minds around the following notion:

var a = 1;
var b = a;
console.log(b); // 1
a = 5;
console.log(b); // 1 (!)

The usual question is, “shouldn’t b be 5, since it is equal to a…?”. And while I had long ago gotten used to the structure of imperative programming, I still find the logic behind this question sincerely reasonable.

For a pair of fresh eyes, the feel in the b = a expression transmits a clear identity logic, conveyed by the one to one construction, which under that same logic should be enduring and persistent through the life of the program. This is the reason behind declarative programming, where the focus is on the structure of the program and the data, rather than the control flow.

So, while I don’t intend to change the nature of JavaScript, I thought it could be interesting to test creating a small framework for constructive associativity between variables in JS.

You can find the source code under https://github.com/garciadelcastillo/X.js


Download the latest version from the distribution folder.

If in a browser, a global X object will be available as a namespace for the library. If in Node.js, you should manually require it via:

var X = require('somepath/x-0.1.0.min.js'); // an actual npm package is wip...

Hello World

X is the main object used to interface with the library, in a fabric-like chainable fashion. To instantiate a new variable, use X.var(value) and access it through the .val property:

var a = X.var(100);
console.log(a.val); // 100
a.val = 150; // 150

Associative variables can be created as children of parent variables, and any changes in the parents propagate to children according to their constructive declaration:

var a = X.var(5),
b = X.var(10);

var add = X.add(a, b);
console.log(add.val); // 15

a.val = 20;
console.log(add.val); // 30

And yes, the hello world example… ;)

var greet = X.var('Hello'),
to = X.var('world');

var greeting = X.concat(greet, ' ', to, '!'); // greeting.val => "Hello world!"

to.val = 'folks'; // greeting.val => "Hello folks!"
greet.val = 'Howdy'; // greeting.val => "Howdy folks!"

Examples / Features


Everything declared via X. is permanently linked to its parents in a constructive logic:

var a = X.var(true),
b = X.var(false),
u = X.var(1),
v = X.var(2),
w = X.var(3);

var and = X.and(a, b), // and.val => false
mul = X.multiply(u, v, w), // mul.val => 6
div = X.divide(u, v); // div.val => 0.5

Associative variables can also spawn characteristic children via their prototypes:

var notAnd = and.not(), // notAnd.val = true
halfMul = mul.half(); // halfMul.val = 3

Changes in parent objects propagate all the way down the hierarchy tree:

b.val = true;
console.log(and.val); // true
console.log(notAnd.val); // false

And changes in objects with parent dependencies are (still) not allowed:

mul.val = 100; // "X.js: Sorry, this variable is constrained"
console.log(mul.val); // 6


Shortcuts to the .val properties of declared XVAR objects can be attached to an object for ease of access, and they will show as capitalized versions of the variable’s name (I know this convention is commonly used to denote constancy, but it is so opposite to the actual meaning that I think it fits… ;)

X.linkVars(window); // capitalized shortcuts are attached to the window object
console.log(W); // 3
W = 10;
console.log(MUL); // 20


X.js is weakly-typed, maintaining the spirit of JavaScript. Variables are declared without explicit type, variable types can change at runtime, and operations are coerced when possible:

var u = X.var(1),
v = X.var(2);

var add = X.add(u, v);
console.log(add.val); // 3

u.val = 'foo';
console.log(add.val); // "foo3"

When necessary, explicit associative type casting is also available:

var u = X.var(1),
v = X.var(2),
uStr = X.string(u); // uStr.val => "1"

var add = X.add(u, v);
console.log(add.val); // "12"

Parents can be any combination of XVARs or primitive objects:

var abc = X.var('abcdefghijklmnopqrstuvwxyz'),
match = X.var('efgh');

var newAbc = abc.replace(match, '[CENSORED]'); // newAbc.val => abcd[CENSORED]ijklmnopqrstuvwxyz
match.val = 'foo'; // newAbc.val => abcdefghijklmnopqrstuvwxyz (no match was found)


Since the return type of most methods is an XVAR object, transformations can be chained:

var s1 = X.var('Foo'),
s2 = X.var('Bar'),
s3 = X.var('Baz');

var chain = X.add(s1, ' ', s2, ' ', s3).slice(1, 10).replace('Bar', s1); // "oo Foo Ba"

Pattern Matching

When working with arrays as primitives, parents are combined at the element level by default, and the matching pattern can be customized:

var chars = X.var(['a', 'b', 'c']),
nums = X.var([0, 1, 2]);

var sum = X.add(chars, nums);

// One to one matching
console.log(sum.val); // ["a0", "b1", "c2"]

// Default matching pattern is 'longest list'
chars.val = ['a'];
console.log(sum.val); // ["a0", "a1", "a2"]

// Changes in array lengths propagate to children
chars.val = ['A', 'B', 'C', 'D', 'E'];
console.log(sum.val); // ["A0", "B1", "C2", "D2", "E2"]

// Matching type can be updated:
console.log(sum.val); // ["A0", "B1", "C2"]

sum.setMatchingPattern('cross-reference'); // aka 'cartesian product'
console.log(sum.val); // ["A0", "A1", "A2", "B0", "B1", "B2", "C0", "C1", "C2", "D0", "D1", "D2", "E0", "E1", "E2"]


Virtually, any kind of child variable can be created by customizing the relation to its parents via the X.compose() constructor:

var radius = X.var(10);

// A composite expression is created passing all parents as arguments,
// plus an update callback function which will be passed an array with
// all the parents' value(s) in ordered sequence.
// Note passed objects are the actual values, so no need to use the .val accessor
var length = X.compose(radius, function(parents) {
return 2 * Math.PI * parents[0];
var area = X.compose(radius, function(parents) {
return Math.PI * parents[0] * parents[0];

// If parents are arrays, individual items will be passed to the callback
// according to the specified matching pattern
var colors = X.var(['blue', 'red', 'green']),
animals = X.var(['dragon', 'turtle']);
var lsd = X.compose(colors, animals, function(parents) {
return "I just saw a " + parents[0] + " " + parents[1] + "!";
console.log(lsd.val); // ["I just saw a blue dragon!", "I just saw a red turtle!", "I just saw a green turtle!"]