WatchableJS

WatchableJS provides the Watchable class which uses Proxies behind the scenes to allow adding Event-like handlers to value changes of any type T. Simply create a new Watchable(T) and add listeners to execute provided callbacks on value changes. Or use the when() member function to invoke a callback only once, based on a predicate; either immediately if the predicate is already true, or the next time the value changes to make it true. The callback and predicate functions optionally take a ChangeEvent parameter which provides data on the change.

Rationale

I wanted the ability to asynchronously await a value change on a js primitive in a very generic way from different scopes. I found I could do it with Promises or rjrx Observables but not without using multiple objects/symbols; I wanted a simpler syntax. Once I got started I got a bit carried away and decided to make it support object types as well.

Installing

npm i @blakecanter/watchablejs

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Constructor

A Watchable instance takes its initial value, x, of type T (or undefined if no initialization parameter is supplied), and wraps it in a Proxy of the object { value: x }, where the type of value is T. However, this is abstracted away by the class's value property getter, which returns the value property directly.

const ex1 = new Watchable(); // ex1.value === undefined
const ex2 = new Watchable("hello"); // ex2.value === "hello"
const ex3 = new Watchable<boolean>(); // ex3.value === undefined
ex3.value = "not boolean"; // error: "Type 'string' is not assignable to type 'boolean'.(2322)"

Properties

Watchable.value

Access to the wrapped primitive/object.

Methods

Watchable.addChangeListener(callback, options)

Add a callback to Watchable.value or nested property changes.

• callback takes an optional changeEvent parameter

• options is an object with the shape:

  { once?: boolean; condition?: PredicateData; }

  • once: if true, the handler will be removed after its first invocation

  • condition: an object with the shape:

    { propertyPath?: string; predicate: PredicateFunction; }

    • propertyPath: optional and only for use with object types. A string mimicking the dot or bracket notation lookup of a nested property. If this successfully resolves, the predicate's changeEvent.res parameter is populated with its value. Ex. "prop1.prop2.prop3" or "['prop 1']['prop 2'].prop3" or 'prop1["prop 2"]["prop 3"]' etc.
    • predicate: a predicate function which takes a changeEvent parameter and must return a boolean (can be coerced) indicating whether the callback should be invoked or not.

Watchable.when(predicateFn, callback)

If predicateFn returns true, immediately invokes callback, otherwise adds a change listener with callback and options.once = true and options.condition.predicate = predicateFn.

Watchable.when(value, callback)

If value === this.value, immediately invokes callback. Otherwise, sets up a one-time change listener with that predicate.

Watchable.when(propertyPath, value, callback)

If [propertyPath resolution] === this.value, immediately invokes callback. Otherwise, sets up a one-time change listener with that predicate.

Watchable.when(propertyPath, predicateFn, callback)

Like the other overloads except predicateFn's changeEvent parameter will have its "res" property populated with the resolution of propertyPath. See the example.

Watchable.promiseWhen(predicateFn)

Watchable.promiseWhen(value)

Watchable.promiseWhen(propertyPath, value)

Returns a Promise that resolves when Watchable.when would trigger.

Watchable.removeChangeListener(callback)

Removes the callback from the instances change listeners. Note, unnamed callbacks (such as arrow functions not given a symbol) are converted to strings via toString() for storage and lookup in the internal callbacks object. Therefore, passing the exact same arrow function should remove it. However, if removal is planned, a named function should be used for adding/removing listeners.

Watchable.clearListeners()

Removes all change listeners from the instance.

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The changeEvent parameter

Both callback and predicate functions optionally take a single ChangeEvent object parameter which provides data on the change.

interface ChangeEvent<T> {
    newValue: any;
    oldValue: any;
    root: T | undefined;
    target?: any;
    property?: string;
    res?: any;
}

newValue

If T is a primitive, its new value. If T is a complex type, this is a new property value, however deeply nested.

const count = new Watchable(1);
count.addChangeListener(e => { console.log(e.newValue); });
count.value++; // logs: 2
count.value += 100; // logs: 102

const foods = new Watchable({
    dairy: ["cheese", "milk", "yogurt"],
    grains: ["oats", "wheat", "barley", "popcorn"]
});

foods.addChangeListener(e => { console.log(`${e.property}: ${e.newValue}`); } );
foods.value.grains.push("corn"); // logs: "4: corn"

oldValue

If T is a primitive, its previous value. If T is a complex type, the previous value of a changed property, however deeply nested. If the property/value is a complex type, then oldValue === newValue because newValue/oldValue are shallow copies. Also, if the property didn't already exists (such as a new array index), oldValue will be undefined.

target

The object whose property changed. Will always be "value" for Watchable primitives (since they are wrapped in { value: x }).

root

Alias for the Watchable.value property (which will be undefined for uninitialized instances and the same as newValue for primitives).

property

The property that got changed.

res

The resolution of the propertyPath string if one was provided via a paired predicateFn, else undefined.

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Examples

The value of whatever type the Watchable was initialized with is accessed through the value parameter:

// Primitive
const myWatchable = new Watchable("hello");
console.log(myWatchable.value); // logs: "hello"
// Object
const watchableObj = new Watchable({ message: "hi" });
console.log(watchableObj.value.message); // logs: "hi"

using addChangeListener()

const message = new Watchable("hello");
message.addChangeListener(e => { console.log(e.newValue); });
message.value = "goodbye"; // logs: "goodbye"

Using when(predicateFn, callback)

const setupComplete = new Watchable(false);
setupComplete.when(e => e.newValue, () => { console.log("setup has completed"); })
setupComplete.value = true; // logs: "setup has completed"

If predicateFn already coerces to true, the callback will fire immediately. Therefore, Watchable.when() a great way to avoid race conditions:

const setupComplete = new Watchable(true);
setupComplete.when(e => e.newValue, () => { console.log("setup has completed"); }) // logs: "setup has completed"

Using when(propertyPath, predicateFn, callback)

You may provide a string resolvable (via . notation) to a nested property (if T is an object) to have changeEvent.res set to that property's value:

const person = new Watchable({
  name: "Blake",
  qualities: {
      titles: ["Farmer", "Application Programmer II"],
      interests: ["programming", "farming"],
      phenotype: {
          hair: "red",
          eyes: "blue",
      }
  }
});
person.when(
    "qualities.interests.length",
    e => e.res >= 5, // condition is met when qualities.interests.length >= 5
    (e) => {
        console.log(`${e.root.name} has quite a few interests,
         including ${e.target.join(", ")}`)
    }
);
person.value.qualities.interests.push("reading");
person.value.qualities.interests.push("hiking");
person.value.qualities.interests.push("meditation");
// logs: "Blake has quite a few interests, including programming, farming, reading, hiking and meditation"