Detailed explanation of Javascript Objects_Basic knowledge

Create object

•Object direct quantity

var o = {
 foo : "bar"
 }
 

•Constructor

var o = new Object();
 

•Prototypal inheritance

var p = Object.create(o);
 

Class inheritance

Javascript objects have their own properties and inherited properties.

•When querying the attribute x of object o, first search for the attribute x in o. If not found, then search for the x attribute in the prototype object of o until x or an object whose prototype is null is found. >

•When assigning a value to the x attribute of object o, if o already has an own attribute x, change the value of x. If attribute x does not exist in o, create an x ​​attribute for o and assign a value

•In other words, the prototype chain will only work when querying.

var O = {
 x : 1
 };
function P() {
 this.y = 2;
 }
P.prototype = O;
var t = new P();
 console.log(t);
 console.log('x' in t);//true
 console.log(t.hasOwnProperty('x'));//false
 

You can use in or hasOwnProperty to determine whether there is a property in the object.

Object properties

•Traverse object properties

You can use for..in to iterate over the properties of an object

When using for..in, the properties on the prototype chain will be traversed. The traversal order is breadth-first traversal

So using hasOwnProperty you can determine whether it is the object's own property.

•Characteristics of object attributes

Use Object.getOwnPropertyDescriptor() to get the descriptor of a specific property of an object

Writable (writable) indicates whether the object properties are writable

For example

var o = {
  foo : 'bar'
}
Object.defineProperty(o, "foo", { writable : false });
o.foo = 'world';
console.log(o.foo);//仍然输出bar

Enumerable indicates whether the object properties are enumerable

For example

The enumerable of attributes such as length in Array is false, so,

for (p in Array) {
  console.log(p);
}

Output nothing

Configurability (configurable) indicates the configurability and enumerability of properties that can be modified

These configuration properties can be defined using Object.defineProperties.

Object.defineProperty(o, "foo", { writable : false });

Get represents the method of obtaining the properties of an object

Set represents a method for setting object properties

Example

var book = {
  _year: 2004,
  edition: 1
};
Object.defineProperty(book, "year", {
  get: function () {
    console.log('get year');
    return this._year;
  },
  set: function (newValue) {
    console.log('set year');
    if (newValue > 2004) {
      this._year = newValue;
      this.edition += newValue - 2004;
    }
  }
});
book.year = 2005;//控制台输出‘set year'
console.log(book.year);//控制台输出‘get year'和year的值

Object methods

toString converts the object into a string. The default conversion will be something like [object Object], so if you need to convert it to json format, you can use JSON.stringify

valueOf is used when objects need to be converted into other types. Again, there's not much to say about the default conversion.

Executable object

An executable object can be created through the following methods

function bar(o) {
  var f = function() { return "Hello World!"; }
  o.__proto__ = f.__proto__;
  f.__proto__ = o;
  return f;
}
var o = { x: 5 };
var foo = bar(o);
console.log(foo());
console.log(foo.x);
console.log(typeof foo);//function

can be used as an object (with a prototype chain), or as a function to be called directly