get.md

Observer.get()

This method is used to read a property. It corresponds to the JavaScript's Reflect.get() function, which is itself the programmatic alternative to the syntax for property access – obj.property; obj[property].

Observer.get() brings the added benefit of triggering interceptors.

• Syntax
• Usage
• Usage as a Trap's "get" Handler
• Usage with Property Getters
• Intercepting Observer.get()
• Related Methods

Syntax

// Read a specific property.
// The return value will be the value of the property
var value = Observer.get(object, propertyName);

// Read a list of properties
// The return value will be a key/value map of the listed properties
var values = Observer.get(object, propertyNames);

Parameters

• obj - an object or array.
• propertyNames/propertyName - the list of properties, or a property, to read.

Return Value

• value - any - the value of the property
• values - object - a key/value map of the listed properties

Usage

Reading a Specific Property

let obj = {
    fruit:'orange',
    brand:'apple',
};

let fruit = Observer.get(obj, 'fruit');

Reading a List of Properties

let obj = {
    fruit:'orange',
    brand:'apple',
};

let fruits = Observer.get(obj, ['fruit', 'brand']);

Usage as a Trap's "get" Handler

Observer.get() can be used as the "get" handler in Proxy traps.

let _obj = new Proxy(obj, {get: Observer.get});
let _arr = new Proxy(arr, {get: Observer.get});

Read operations will now be forwarded to Observer.get() and interceptors that may be bound to the object will continue to respond.

let fruit = _obj.fruit;
let value = _arr[2];

Usage with Property Getters

It is possible to implement property getters that use Observer.get() behind the scene. This gives us the benefit of using JavaScript's property access syntax while still driving interceptors.

This is automatically done by the Observer.init() support function.

// Virtualize a property or multiple properties
Observer.init(obj, 'fruit');
Observer.init(obj, ['fruit', 'brand']);

// Now we can do without Observer.get
let fruit = obj.fruit;
let brand = obj.brand;

We could follow the pattern above for arrays; we could even init an array's prototype methods instead. The specific keys modified after calling these methods will be announced to observers.

// Virtualize the arr.push() and arr.splice() methods
Observer.init(arr, ['push', 'splice']);

// Now we can do without Observer.set
arr.push('Item 1');
arr.push('Item 2');
arr.splice(1);

Intercepting Observer.get()

Using Observer.intercept(), it is possible to intercept calls to Observer.get(). When a "get" operation triggers an interceptor, the interceptor will receive an event object containing the property name to read.

Observer.intercept(obj, 'get', (event, recieved, next) => {
    // What we recieved...
    console.log(event.name);
    // The read operation
    return obj[event.name];
});

let value = Observer.get(obj, 'fruit');

When the "get" operation is of multiple properties, the interceptor gets fired for each property while also recieving the total list of properties as a hint - via event.related.

Observer.intercept(obj, 'get', (event, recieved, next) => {
    // What we recieved...
    console.log(event.name, event.related);
    // The read operation
    return obj[event.name];
});

let values = Observer.get(obj, ['orange', 'apple']);

The above should trigger our interceptor twice with event.related being ['fruit', 'brand'].

Related Methods

• Observer.observe()
• Observer.intercept()