Neglected Pizza Money


    1.1.2-b • Public • Published v1.1.2 beta

    Introduction to Watchlight

    A light-weight, comprehensive, reactive framework for business logic and when things change.

    Watchlight provides a range of approaches to support reactive programming beyond the DOM and user interface with a light-weight JavaScript module (14K minified, 4.6K gzipped).

    • Event listeners on any reactive object via addEventListener.
    • Observers via functions wrapping reactive objects, e.g. observer(() => console.log( will log the name every time it changes.
    • A range of Observable capability similar to RxJs.
    • Inference rules similar to Drools or Rools and modeled after the Promise paradigm.
    • Spreadsheets ... no reactive library would be complete without them.

    The spreadsheet and rules are provided as separate files, rule.js and ./sheet.js which are not included in the 4.5K size stated above. They are XXX and YYY respectively.

    Watchlight does not use any intermediate languages or transpilation; hence, you can debug all of your code as written using a standard JavaScript debugger.


    Watchlight is provided as a JavaScript module. It can be loaded directly in a modern browser or used directly in NodeJS.

    npm install watchlight

    The repository is at

    Transpiling and minifying is left to the developer using the library.

    Using The Examples

    There are examples in the examples directory and sub-directories. Most examples can be run by both loading an HTML file and running the command node examplefilename.js. The HTML files just load the same JavaScript files that are fed to NodeJS on the command line.

    Psuedo Classes

    Watchlight makes extensive use of Proxy or other constructs around objects and functions you provide. These constructs supplement the behavior of your functions and classes, but instanceof will only be true for your original symbols, i.e. nothing will ever be an instanceof a psuedo-class.

    The psuedo-classes include:

    • Observable
    • Observer
    • Subscription
    • Rule
    • Partial
    • Sheet
    • Dimension
    • Cell

    Observable Objects, Constructors and Functions

    The core of Watchlight is the psuedo-class Observable. Observable objects are reactive, they drive application in a non-procedural manner. They can have event listeners and subscribers attached, be the subject of observers, and be referenced by inference rules.

    When the properties of Observable objects contain sub-objects, the sub-objects are returned as reactive objects when accessed.

    If a class is made Observable and observeInstances:true is passed as an option, it will return an Observable instance when it is called to create a new instance.

    Reactive Object API


    Observable Observable(target:object|function [, {global:boolean, observeInstances:boolean}])

    Wraps target with a Proxy and makes it Observable, i.e. a subject of Subscriptions, Observers, and Rules.

    Setting global to true when the type of the target is a function will make the function available in the globalThis context.

    Setting observeInstances to true will automatically make any instances created by a class constructor Observable. Note, this will only work for classes defined using class <className> {}, not old style JavaScript classes.

    There are actually some other options, but they are for internal use and remain un-documented for now.

    Returns: Observable (Remember, this is a pseudo-class. It is a Proxy around the target.)


    Watchlight supports many of the functions of RxJs, but is also supports a somewhat simpler reactive concepts, the Observer. Observers are functions that get invoked automatically every time the properties on the Observeable objects they reference change in value. They are more powerful that event listeners because they can operate across multiple objects.

    Those familiar with RxJs can think of observers as functions that automatically subscribe to an Observable when the Observable detects that the observer accesses some of its properties. What is super powerful about Observer is that it will automatically subscribe across multiple Observables.

    Observers are the cornerstone of the watchlight spreadsheet functionality. A slimmed down version is also used in the Lightview reactive UI library.

    Observer Examples
    const user = Observable({name: "mary"});
    const hello = Observer(() => {
    }) = "joe";


    Hello mary
    Hello joe

    Nested property access automatically creates child reactors, changes to which will invoke the observer so long as the changes are made via navigation through the Observable user.

    const user = Observable({name: "mary", contactInfo: {phone: "555-555-5555"}});
    Observer(() => {
        console.log(JSON.stringify(user)); // recursively accesses every property
    }) = "999-999-9999";



    You can call an Observer directly:

    const user = Observable({name: "mary", contactInfo: {phone: "555-555-5555"}}),
        logUser = Observer(() => {
            console.log(JSON.stringify(user)); // recursively accesses every property



    Observer API

    Observer Observer(aFunction:function [,thisArg:object,...args:any])

    Creates an Observer from aFunction you provide. The Observer will be called any time the properties on the objects it references change in value. You can also call the Observer directly like it was the original function.

    Observers are indexed internally by name. Creating an observer from a function with the same name as a previous observer will overwrite the old observer. Anonymous functions are not overwritten.

    You can pass a default thisArg and ...args when creating an Observer.

    Synchronously invoked sub-functions that access Observable data will create reactive dependencies that can cause invocation of the Observer at a later time. Use unobserve to avoid creating a reactive dependency. Asynchronously invoked sub-functions, i.e. those inside setTimeout will not cause reactive dependencies.

    import {reactive, observer} from "../../watchlight.js";
    const user = reactive({name: "mary"});
    const hello = observer(() => {
    const world = reactive({});
    observer(function (message) {
        this.user =
    }, world, "Welcome to the world")
    observer(() => {
        if (world.user) console.log(`${world.user} owns the world.`)
    }) = "joe";


    Hello mary
    Welcome to the world mary
    mary owns the world.
    Hello joe
    joe owns the world.
    Welcome to the world joe
    Hello joe

    void observer.stop()

    Stops observer from executing when the objects it references change.

    void observer.start()

    Restarts the observer so it will respond when the objects it references change.

    Observer observer.withOptions( {onerror:function} )

    Observer error handling defaults to re-throwing errors thrown by wrapped functions. This can be changed to swallow the error by passing {onerror:()=>{}} or use the error as the value by passing {onerror:(e) => e}.

    any unobserve( aFunction:function )

    You can nest unobserve inside an observer if you do not want changes to a particular object or property to cause invocation of the observer.

    Functions wrapped in unobserve are transient and will get the this context of the enclosing observer so long as you define them using =>.

    Returns: The value returned by the function you provide.

    unobserve is useful when you need to use arrays but do not want index modification or access to cause an observer to be re-invoked or when you want to use JSON.stringify.

    import {reactive, observer, unobserve} from "../../watchlight.js";
    const tasks = reactive([
        {name: "task1", duration: 2000},
        {name: "task2", duration: 3000},
        {name: "task3", duration: 1000},
        {name: "task4", duration: 2000}]);
    const doTasks = observer(() => {
        const task = tasks.currentTask = unobserve(() => tasks.shift());
        if (task) {
            // complete the task in the defined duration
            setTimeout(() => task.complete = true, task.duration);
            // will access all properties
            console.log("doing:", unobserve(() => JSON.stringify(tasks.currentTask)));
            observer(() => {  // called whenever current task completion is changed
                if (tasks.currentTask?.complete) {
                    // will access all properties
                    console.log("completed:", unobserve(() => JSON.stringify(tasks.currentTask)));
        } else {
            console.log("Waiting for more tasks ...");
            const interval = setInterval(() => {
                if (tasks.length > 0) { // poll for length change, not reactive since in setInterval
    setTimeout(() => tasks.push(reactive({name: "task5", duration: 2000})), 10000);
    Subscriptions and Event Listeners

    Reactive objects created using Observable(target) can dispatch event listeners similar to those used in web browsers.

    Event listeners are added via subscribe. They can be revoked via unsubscribe. If the target supports addEventListener, e.g. if you make a DOMElement Observable, then addEventListener is also supported. Listeners are indexed internally based on their text representation; hence, if you plan to overwrite them, you should not use functions that contain closure values and count on the functions being preserved as different event handlers.

    Subscription subscribe(subscription: function|string|ObservableEventDescriptor, target:Observable)

    You will usually pass a function as the value for subscription when subscribing. The name of the function should be the event type you wish to subscribe to. If you pass an un-named function, it will be invoked for all events.

    Passing a string for subscription is only useful for pipelined subscriptions and is covered in more detail elsewhere.

    The function or string passed is actually just a shorthand for an ObservableEventDescriptor which has the surface {eventType: string, listener: function}.

    class Person {
        constructor({name, age}) {
   = name;
            this.age = age;
    Person = Observable(Person);
    const joe = Person({name: "joe", age: 27}); // joe is a reactive object
    subscribe(function change({target,property,value,oldValue}) {
        console.log(`${}'s ${property} is changing from ${oldValue} to ${value}`)

    Functions can be made into Observables. When they are, you can subscribe to the invocation.

    function helloWorld() {
        console.log("Hello world!");
    subscribe(function apply({target,thisArg,argsList}) {
        console.log(`${} is about to execute`);

    You may have noted from the above that subscriptions are notified prior to an activity occuring, this allows the activity to be cancelled just like DOM events.

    subscribe(function change({target,property,value,oldValue,preventDefault}) {
        if(! {
            alert(`You can only change your own name!`);

    That's the basics, we cover more advanced use of Subscriptions later.

    void observableInstance.addEventListener( eventType:string, listener:function, options:Object)

    Only available if the target of the observableInstance supports addEventListener like a DOM Element.

    Adds a function as an event listener on the eventName. The listener will receive an ObservableEvent when the eventName occurs on the observableInstance, i.e. the listener has the signature ({event,}).

    The options argument has the surface {synchronous,once}.

    Returns: void. If you want chaining, use subscribe.

    boolean observableInstance.hasEventListener( eventType:string, listener:function)

    Only available if the target of the observableInstance supports addEventListener like a DOM Element.

    Checks for existence of function with the same string representation as a listener for eventName on the reactiveObject.

    Returns: The true or false.

    boolean observableInstance.removeEventListener( eventType:string, listener:function)

    Only available if the target of the observableInstance supports removeEventListener like a DOM Element.

    Removes a listener for eventName with the same text representation as listener.

    Returns: The true if the listener was found and removed, otherwise false.



    An object with the string property type containing an event name, e.g. {type:"change"}. Other properties vary based on event type and may include:

    • target - the reactive proxy generating the event
    • currentTarget - the target or object further up the tree as a result of bubbling
    • property - the property impacted on the target
    • value - the current value of the property
    • oldValue - the previous value of the property before the event

    Typically, ObservableEvents are created automatically by watchlight, rather than by an application developer. However, it is possible to add custom event types.

    Events will bubble up from an object to its containing objects. For the data below, subscribers registered on object will get events for changes to

    const object = reactive({person: {name: "joe", age: 27}}),
        aPerson = object.person; = "mary";

    The ObservableEvent API is very similar to the browser Event API. However, unlike DOM nodes, regular objects can be contained by multiple parents; hence, bubbling can propagate more widely.

    void observableEvent.preventDefault()

    Prevents the event type from occurring. For example, if there is a change Subscription (a.k.a. listener) calling preventDefault will stop the change from occurring. The event will still bubble.

    void observableEvent.stopPropagation()

    Stops bubbling to parent objects, but all subscribers on the current object will continue to get the event.

    void observableEvent.stopImmediatePropagation()

    Stops bubbling when called from a subscription, and all subsequent subscriptions will be blocked.

    Interface ObservableEventDescriptor

    {eventType:string, listener:function}

    The built-in event types are described below. Also see custom event types.


    A wild card that will match any event.


    Listeners on the event name apply are invoked when an Observable function is about to execute.


    Listeners on the event name change are invoked whenever a property value is changing on an Observable.


    Listeners on the event name defineProperty are invoked whenever a new property is being defined on an Observable. A new property is assumed if the previous value of a property is undefined.


    Listeners on the event name delete are invoked whenever a property is deleted from an Observable.


    A special event supported by Inference Rules when their conditions are satisfied.


    A special event supported by Observables that have been asserted for use by Inference Rules. Fires when the object is being removed from imnstances tracked by its constructor.

    Event Listener Example

    const aPerson = reactive({name: "joe", age: 27});
    aPerson.addEventListener("defineProperty", ({type, target, reactor, property, value}) => {
        console.log(type, target);
    aPerson.addEventListener("change", ({type, target, reactor, property, value, oldValue}) => {
        console.log(type, target);
        function myDelete({type, target, reactor, property, oldValue}) {
            console.log(type, target);
        {synchronous: true});
    aPerson.married = true; // invokes the defineProperty handler asynchronously using setTimeout
    aPerson.age = 30; // invokes the change handler asynchronously using setTimeout with the oldValue as 27
    delete aPerson.age; // invokes the delete handler synchronously with the oldValue as 30 (due to the change above)
    aPerson.removeEventListener("change", ({type, target, reactor, property, value, oldValue}) => {
        console.log(type, target);
    aPerson.removeEventListener("delete", "myDelete"); // removes the delete event listener
    aPerson.removeEventListener("delete", function myDelete() {
    }); // also removes the delete event listener

    Custom Event Types

    You can add custom event types by using Reactor.registerEventType(eventName). You can then add and use event listeners that will automatically get invoked and support the standard API when events are posted using reactiveObject.postMessage(eventName,options={}).

    Advanced Subscription Use

    Subscriptions support the routing and piping of events.

    The below watches for clicks on a button, ignores clicking faster than 1 every second, delays 5 seconds and logs the click to the console.

    const observableInstance = Observable(document.getElementById("mybutton"));
    subscribe("click" ,observableInstance)
        .subscribe((event) => {

    The function subscribe knows to expect an Observable as the second argument. So, unless you need to reference your Observable elsewhere, you can shorten the above code and the Observable will be automatically created.

    subscribe("click" ,document.getElementById("mybutton"))
        .subscribe((event) => {

    Above pipe is a method on Subscription returned by subscribe. Watchlight also exposes pipe and route as top level functions. So, you can make your code even shorter.

        .subscribe(click(event) => {

    And, since you are wrapping a DOM Element, you can use addEventListener if you prefer.

        .addEventListener("click",(event) => {

    Watchlight also supports route, which behaves the same way as most event or http routers with middleware.

            .route(({target}) => target.value==="joe", ({target}) => ... do something)
            .route(({target}) => target.value==="mary", ({target}) => ... do something)
            .route(() => throw new TypeError(`${target.value} must be mary or joe`))

    Routes are committed to once the first function in the route succeeds. Then the remaining functions are called until one returns undefined or calls a preventDefault, stopPropagation, or stopImmediatePropagation on the event it gets as an argument.

    Under the hood, pipe just creates a single route and then locks the subscription so that no more routes can be added.

    You will find many of the same pipeline operators as provided by RxJs, e.g.

    count, debounce, delay, filter, timeThrottle, map, etc. There are also some additional operators, e.g. sum, average. Since the list is long and each requires its own explanation, they are provided in a separate file so that we can move on to inference rules.

    Inference Rules

    Inference rules can match across multiple objects up and down the inheritance hierarchy. They can chain across multiple then and catch statements similar to Promises. These chained statements can add new objects or change existing objects. Rules also respond to the addition and removal of new objects in a prioritized manner. Objects can even be automatically removed if data changes and the rules that created the objects no longer have their conditions satisfied.

    To avoid the creation of a special language or the representation of operators like "==" and ">" as strings, the inference engine does not use the Rete Algorithm or a derivative like most rule engines. However, it is small (4K minified/gzipped) and fast. And, this means you can use the JavaScript debugger to step through all of your code as it is written.

    Watchlight is currently in beta, but tests on an 8 MB Ryzen 4000 5 show that 250,000+ rule tests can be processed per second in Firefox, Chrome, Edge and NodeJS, even when the potential rule matches exceed 1 million combinations of objects. The number of rules that actually fire per second is entirely dependent on the nature of the logic being modelled. If no rule conditions are satisfied, no rules will fire! Head-to-head comparisons of different rule processing engines can only be made using the same rule and data sets.

    Anatomy of A Rule

    Rules consist of:

    • condition - A single synchronous function that accepts one object as an argument and must return true or false. The property names effectively represent variables in the condition. The values of the properties must be instances created from classes defined using class <classname> { }. Conditions should be side effect free. Create new objects or call non-synchronous or side effect producing functions in conditions at your own risk.
    • domain - An object with the same properties as the argument to condition. The values of the properties are the expected classes of the values in the condition argument.
    • options (optional) - Configuration data for the rule.
    • actions - A series of chained then statements, the first of which gets the same argument as the condition. Subsequent actions get the return value of the preceding action as their arguments. Chaining stops when an action returns undefined.
    • exception handlers - One or more catch statements interspersed with actions, although usually just the last statement.
        ({person1, person2}) => { // start condition
            return !== &&
                typeof (person1.age) === "number" &&
                typeof (persone2.age) === "number"
        }, // end condition
        {person1: Person, perrdon2: Person},// domain
        {priority:10} // options
        .then(({person1, person2}) => { // first action
                return {person1, person2, avgAge: person1.age / person2.age}
        .then(({person1, person2, avgAge}) => { // chained action
            console.log(,, avgAge)

    Rule Processing

    Rules are processed in a cycle with a run limit that may be Infinity:

    1. Match all rules to all combinations of objects in rule accessible memory, a.k.a. "working memory", by rule domain
    2. Add matched rules to a rule agenda
    3. Sort rule agenda by rule priority
    4. For each rule on the agenda
      1. For each combination of objects
        1. remove combination from combinations
        2. Test the condition with the combination
          1. if failed, goto next rule
          2. else fire rule and process actions (add, modify, remove objects, call functions)
            1. if action adds a higher priority rule to agenda goto 3
            2. else goto next combination
      2. No more combinations goto next rule
    5. No more rules
      1. if runlimit exceeded, stop
      2. else set timeout to watch for new rules added to the rule agenda

    Rule Examples

    import {when,Observable} from "./rule.js";
    class Person {
        constructor({name,age}) {
            if(name==null || age==null) throw new TypeError("Person requires both name and age")
   = name;
            this.age = age;
    Person = Observable(Person);
    when(({object}) => true, {object: Object})
        // runs every time a new Object is added or changed
        .then(({object}) => console.log(object))
    new Person({name: "joe"});


    Person {name:"joe"}

    Note the import of Observable from rule.js rather than watchlight.js. This version of Observable has been enhanced to support rule processing. Specifically it ensures the Observable classes keep track of all their instances in a manner that makes rules the most efficient. It also enables the creation of pseudo-class Partial objects (see below).

    when(({person}) => person.age < 21, {person: Person})
        // runs every time a new person is added with an age < 21
        // or a person's age changes to < 21
        .then(({person}) => console.log(person, "is a minor"))
    Combo = Observable(Combo);
    when(({person1, person2}) => {
            // creates pairs of people, automatically removes pair 
            // if a person's name changes or a person is removed
            // Combo has an equals methods on it so that it is reflexive
            return !== && not(Combo(person1, person2));
        }, // then, create pair
        ({person1, person2}) => {
            return this.justifies({person1,person2},new Combo(person1, person2))
        .then((combo) => console.log("A pair:", combo))

    Note the use of Combo without the word new in the rule test. Combo will test like it is an instance of Combo, but it is not created with the Combo constructor. Watchlight support a concept called Partials, which are partially populated instanced of classes that will not throw construction errors or trigger other rules.

    Rules API

    any rule.catch( errorHandler:function )

    errorHandler has the call signature (error:Error).

    If the error errorHandler returns undefined, the error will be swallowed.

    If the errorHandler returns anything else, it will be used as the input argument to the next action in the chain.

    If the errorHandler throws, the next catch statement will be sought.

    boolean exists( object:Object [,test:function] )

    Checks to see if an object or partial object exists. Typically, used as part of rule condition.

    let joe = reactive(new Person({name: "joe", age: 20})),
        mary = reactive(new Person({name: "mary", age: 27})),
        joe = assert(joe);
    // true
    joeexists = exists(joe);
    // true because of joe
    namedjoeexists = exists(Person({name: "joe"})); // a Partial not really a Person, will not throw error or trigger rules
    // false because joe is 20 and mary is not asserted
    rightageexists = exists(Person({age: 21}));
    // false, because mary was not asserted to rule memory
    namedmaryexists = exists(Person({name: "mary"}));
    // true, because a Person that has all the same properties and values, i.e. mary, exists
    deepequalexists = exists(Person({name: "mary", age: 27})); 

    any rule.then(action:function)

    action has the call signature (data:any).

    data is typically an object with multiple properties the values of which are other objects, e.g.

        person:Person({name: "joe", age: 27}), 
        table: Table({number: 12, capacity: 10})

    If the action returns undefined, action processing will cease.

    If the action returns anything else, it will be used as the input argument to the next action in the chain.

    Inside the action function you can use:

    this.justifies(justification,conclusion) where justification is an object holding the facts that must remain constant for the facts in the conclusion to remain in place. See the examples/rules/diagnostic-confidence.js

    Returns: the return value of action.

    boolean not( object:Object )

    A convenience, equivalent to !exists(object).

    boolean retract( object:Object )

    Stops the object from being tracked by the Observer class that created it. As a result, rules will not have access to it and any objects created in the scope of justifies, where the object was part of a justification will be removed.

    Returns: Reactive true if the object was being tracked, i.e. was not previously retracted. Otherwise, false,

    Rule when(condition:function, domain:Object [,{priority:number, confidence:float])

    The condition can be an anonymous or named function. The call signature of condition is (object:Object) where object must be an Object with one or more properties. The condition MUST return true or false indicating if the members of the object satisfy the rule conditions.

    The domain MUST be an Object with the same properties as the object argument to condition. The values of the properties MUST be classes or constructors.

    confidence sets a confidence on a rule or Observable data. This is available to the this.justifies function and also via this.withConfidence in the then statements of a rule. a confidence = minimum confidence of data used to fire the rule * confidence of the rule. You can run the example diagnostic confidence or view its source.

    Returns: Reactive Proxy for condition, i.e. a Rule.

    Instance Bound Rules

    Reactive objects can have instance bound rules associated with them in addition to event handlers and observers. Unlike event handlers and observers, these rules get added to the rule processing agenda.

    There are two options for binding. The first is to provide a rule that applies only to the object it is bound to:

    const joe = assert(new Person({name: "joe", age: 27}));
    joe.when((joe) => joe.age > 27)
        .then((joe) => console.log("joe too old", joe.age));

    Note the lack of domain and the un-parametrized object as an argument.

    If Joe's age changes before the rule has an opportunity to fire (perhaps due to a higher priority rule), then the console message will not be written.

    The second option is to allow comparing with other objects:

    const joe = assert(new Person({name: "joe", age: 27}));
    // This rule will match Joe with all possible partners.
    joe.when(({bound, partner}) => {
        return !==
    }, {partner: Person})
        .then(({bound, partner}) => {
            console.log("joe partner", partner)

    Note the domain and the parameterized object as an argument.

    The property bound MUST be present in the condition argument. And, MUST NOT be present in the domain.

    Rule Example Files

    Fibonacci sequence generation: source.

    Pair matching beyond the examples in this document.: source.


    Spreadsheet like functionality is provided through a separately loaded module ./sheet.js. The functionality is headless and depends on object access paths for its notation. It is also n-dimensional and sparse. Formulas can be set at any level in a sheet's data hierarchy and any legal property names can be used for navigation through the hierarchy. Any type of data can be stored in cells. There is no support for selecting, cutting, pasting, etc.; although, these could be provided by a wrapper.

    Dimension and Cell

    Dimension is a psuedo-class, i.e. you can't use instanceof to check if something is a Dimension. Any time an undefined property or sub-property is accessed on a Sheet a Dimension is created. If a Dimension is directly assigned a value or a function, it is converted into an instance of the psuedo-class Cell. Cells only exist at leaf nodes of Dimensions. Existing Dimensions can be overridden and converted into a Cell by direct assignment of a value or function.

    Cells in a Sheet with functions assigned, provide a method withFormat that can take either a string or a function as an argument. If a string, then it should be an un-interpolated string template literal that accesses this.valueOf(). If a function, it will get the cell as its this value, so it can call this.valueOf(). It should return a string.

    The code below can be run or viewed in the examples directory.

    import {Sheet} from "../sheet.js";
    const sheet = Sheet();
    sheet.A[0]; // no assignment is made, so sheet.A[0] will automatically be a Dimension when accessed
    sheet.A[1] = 1; // sheet.A[1] is a Cell. Dimensions and Cells are created automatically
    sheet.A[2] = 1;
    sheet.A[3] = () => A[1] + A[2]; // Note, there is no need to include sheet; watchlight manages the resolution
    sheet.A[4] = 1;
    sheet.B[1] = () => sum(values(A, 2, 3));
    sheet.B[2] = () => sum(A);
    console.log(sheet.B[1].valueOf()); // logs 3
    console.log(sheet.B[2].valueOf()); // logs 5
    console.log(sheet.A[3].valueOf()); // logs 2
    console.log(sheet.A[3].format()); // logs $2.00
    sheet.A[2] = 2;
    console.log(sheet.A[3].valueOf()); // logs 3
    sheet[1][2][1] = () => {
        return A[3] + 1
    }; // completely different dimension approach
    console.log(sheet[1][2][1].valueOf()); // logs 4
    sheet.A[2] = 4;
    setTimeout(() => { // let recalculation settle out
        console.log(sheet.A[3].valueOf()); // logs 5
        console.log(sheet.A[3].format()); // logs $5.00
        console.log(sheet[1][2][1].valueOf()); // logs 6
        console.log(sheet.B[1].valueOf()); // logs 9

    Sheet functions behave like their similarly named counterparts in MS Excel and Google Sheet.

    Most functions will automatically convert cell references to iterables when necessary.

    Some functions require a cell or a value for an argument and not a Dimension. If you call a function that can't take a Dimension with a Dimension, you will get an error similar to this:

    TypeError: isnumber(A.5) 'A.5' is a Dimension not a value or Cell

    Self Referencing Formulas

    Directly circular formulas are automatically avoided by excluding the cell in which the formula is defined from the range it may reference, e.g.

    const sheet = Sheet();
    sheet.tab1.A[1] = 1;
    sheet.tab1.A[2] = 2;
    sheet.tab1.A[3] = 3;
    sheet.tab1.A[4] = () => sum([tab1.A]); // 5


    The path to a Dimension or Cell is available as a property:

    const sheet = Sheet();
    console.log(sheet.tab1.A[1].path); // logs "tab1.A.1"

    Logical and Info Functions

    number count(values:Array|Dimension,{start:number|string,end:number|string})

    number counta(values:Array|Dimension,{start:number|string,end:number|string})

    any iff(test:truthy, value1:any, value2:any)

    boolean isdimension(value:any)

    boolean isblank(value:any)

    boolean isboolean(value:any)

    boolean isempty(value:any)

    boolean islogical(value:any)

    boolean isnumber(value:any)

    boolean isobject(value:any)

    boolean isstring(value:any)

    boolean isundefined(value:any)

    number len(value:any)

    Throws TypeError if value does not have a length or size property or function.

    Math Functions

    number average(values:array|Dimension,{start:number|string,end:number|string})

    number exp(value:number,power:number)

    number log10(value:number)

    number max(values:Array|Dimension,{start:number|string,end:number|string})

    number median(numbers:Array)

    number min(values:Array|Dimension,{start:number|string,end:number|string})

    number product(values:Array|Dimension,{start:number|string,end:number|string})

    number stdev(values:Array|Dimension,{start:number|string,end:number|string})

    number sum(values:Array|Dimension,{start:number|string,end:number|string})

    number variance(values:Array|Dimension,{start:number|string,end:number|string})

    number zscores(values:Array|Dimension,{start:number|string,end:number|string})

    Returns Array.

    Trigonometry Functions

    number acos(value:number)

    number acosh(value:number)

    number asin(value:number)

    number asinh(value:number)

    number atan(value:number)

    number atan2(value:number)

    number cos(value:number)

    number cosh(value:number)

    number pi()

    number rand()

    number sin(value:number)

    number tan(value:number)

    number tanh(value:number)

    Coercion Functions

    number int(value:string|number)

    number float(value:string|number)

    string lower(value:string)

    Array numbers(source:Array|Dimension, start:number|string, end:number|string)

    source can be an Array or a Sheet dimension. If end is less that start the return value is reversed.

    Returns an array of all numbers from the object based on the keys between and including start and end.

    Array numbersa(values:Array|Dimension, {start:number|string, end:number|string})

    source can be an Array or a Sheet dimension. If end is less that start the return value is reversed.

    Returns an array of all values coercible into numbers from the object based on the keys between and including start and end. Strings are parsed as floats and booleans are converted to 1s and 0s.

    string upper(value:string)

    number value(data:any)

    Array values(values:Array|Dimension, {start:number|string, end:number|string})

    source can be an Array or a Sheet dimension. If end is less that start the return value is reversed.

    Returns an array of values from the object based on the keys between and including start and end.


    Watchlight is dual licensed:



    A custom commercial license. Contact

    Change History

    Reverse Chronological Order

    2022-04-25 v1.1.2b Optimized subscribe, made more conformant with RxJs. Deleted kitchensink example since there are now many more structured examples and it was out of date with API. Many RxJs operators added and unit tested, but yet to be documents. This release has seen some performance degradation in rules. Should be able to optimize back in.

    2022-04-20 v1.1.1b Modified naming to be more consistent with RxJs. Added a range of RxJs operators. Corrected issue where bubbles and defaultPrevented were ignored on events. Split rule functionality into a separate file. Deprecated whilst for simpler justifies approach. Optimized rule processing with some light-weight indexing to support faster retract and not. Eliminated assert, creating an Obervable object automatically invokes the rules it may match. Eliminated withOptions on rules to simplify API. Modified Sheet so that closures can ultimately be supported. This required changing the way default tabs can be established for cell formula references. Added many functions for use in formulas.

    2022-04-03 v1.0.17b Updated license token to more standard form.

    2022-04-03 v1.0.16b Added automation of existing tests to package.json.

    2022-03-27 v1.0.15b Modified event bubbling to be consistent with browser approach. preventDefault() will no longer stop bubbling. Use stopPropagation() or stopImmediatePropagation() to stop bubbling.

    2022-03-27 v1.0.14b Fixed issue with ReactorEvent properties not being enumerable, which prevent spread and assign copying.

    2022-03-27 v1.0.13b Support for custom event types added.

    2022-03-26 v1.0.12b More rule examples. Added foundation for confidence based, a.k.a. "fuzzy", reasoning. Modified result portions of whilst for more flexible results return. Adjusted TOC layout and scrolling.

    2022-03-25 v1.0.11b Documentation content updates.

    2022-03-25 v1.0.10b Documentation content updates. Improved swipe behavior of TOC.

    2022-03-25 v1.0.9b Documentation content updates. Renamed main entry point to watchlight.js. More unit tests and event bubbling work. Fixed issue with observers not stopping when requested.

    2022-03-25 v1.0.8b Documentation layout. Added event bubbling. Renamed event property in ReactorEvent to type.

    2022-03-24 v1.0.7b Documentation layout. More unit tests. Fixed issues with checking presence of and removing event handlers.

    2022-03-24 v1.0.6b Documentation TOC tray added.

    2022-03-23 v1.0.5b Documentation style updates.

    2022-03-23 v1.0.4b More unit tests. Documentation content and style updates.

    2022-03-23 v1.0.3b Unit tests, fixed bug in proxy property lookup that was creating extra reactive sub-objects when value was false. Minor rule performance improvement. Added observer.withOptions.

    2022-03-22 v1.0.2b Documentation updates.

    2022-03-22 v1.0.1b Documentation updates, added Partial and Sheet

    2022-03-20 v0.0.7b Documentation updates, observer examples, renaming of some internals

    2022-03-19 v0.0.6b Documentation updates, examples added, enhanced whilst with onassert

    2022-03-19 v0.0.5b Documentation updates, enhanced instance bound rules

    2022-03-19 v0.0.4b Documentation updates, performance improvements

    2022-03-18 v0.0.3b Documentation updates

    2022-03-18 v0.0.2b Documentation updates

    2022-03-18 v0.0.1b Initial public release


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