javaScript basics summary (four)

1, Object

 　　The concept: objects can {...} and contains several optional attributes to create the curly braces. When a property key-value pair, the key is a string value may be any type.

       Creating objects

. 1  the let = User new new Object (); // constructor Syntax
 2 the let User = {}; // "literal" grammar

　　Object attributes:

. 1  the let = {// User object
 2       name: "John", // key "name", the value of "John"
 . 3       Age: 30 // "key" age ", a value of 30
 . 4  };
 . 5  
. 6 attributes key, in front of the colon ":" the values on the right side of the colon

    Adding and deleting objects properties

1  user.sex = "man"; // add properties of objects
 2 the Delete user.sex; // object deletion

　　Use square brackets of the object

1  // word processing multiple attribute
 2  the let User = {};
 . 3  
. 4  User [ "like Birds"] = to true; // set
 . 5  
. 6  Alert (User [ "like Birds"]); // use to true
 . 7  
. 8 Delete User [ "like birds"]; // delete

　　Computed Property

1  // Example a:
 2      the let Fruit = "Apple";
 . 3      the let = {Bag
 . 4              [Fruit]:. 5
 . 5     };
 . 6      Alert (bag.apple);
 . 7  
. 8  // two examples:
 . 9      the let Fruit = "Apple" ;
 10      the let Bag = {};
 . 11      Bag [Fruit] =. 5;
 12 is  // three examples of
 13 is      the let Fruit = "Apple";
 14      the let Bag = {
 15          [Fruit + "Test"]:. 5;
 16      };
 . 17      Alert (bag.fruittest);

　　Shorthand property value

　　When the key and the same attribute values ​​may be written as follows

User = {the let 
    name, name and where // name: the same name 
    Age: 13 is 
}

　　Check the existence of attributes like value

　　Analyzing manner by acquiring attribute is undefined if not present, is used in another way keywords, but there are special circumstances. Specific examples are as follows:

. 1  the let = {User);
 2  
. 3  Alert (user.isExist == undifined) // to true indicates that the property does not exist
 . 4  
. 5  Alert ( "ISEXIST" in User); // attribute to false absent
 . 6  
. 7  // special circumstances
 8  
. 9  the let {obj =
 10      Test: undefined
 . 11  }
 12 is  
13 is  Alert (obj.test == undefined) // to false, but this attribute is present, so that in this way not appropriate when using
 14   Alert ( "Test" in obj) ; // true indicates that the property exists.

　　for ... in loop

 1 let user = {
 2   name: "John",
 3   age: 30,
 4   isAdmin: true
 5 };
 6 
 7 for(let key in user) {
 8   // keys
 9   alert( key );  // name, age, isAdmin
10   // 属性键的值
11   alert( user[key] ); // John, 30, true
12 }

　　Reference copy

. 1  the let = {User name: 'John'};
 2  
. 3  the let ADMIN = User;
 . 4  
. 5  admin.name = 'Pete'; // change the reference of "admin"
 . 6  
. 7  Alert (the user.name); // 'Pete ', Changes are at the Seen from "the User" Reference
 8 above example shows there is only one object, like our drawers have two keys, if a key (admin) to use the 
drawer, use another key later ( user) is open, you will see a change.

　　Comparison of reference

　　

1  // two references point to the same object when they are equal.
2  the let A = {};
 . 3  the let B = A; // copy reference
 . 4  
. 5  Alert (A == B); // to true, both variables refer to the same object
 . 6  Alert (B === A); // to true
 7  
8  // If it is two different attributes, they are not equal, even if empty.
. 9  the let A = {};
 10  the let B = {}; // two separate objects
 . 11  
12 is Alert (A == B); // to false

　　Const object

1  // const be a modified object can be modified.
2  const = {user
 . 3    name: "John"
 . 4  };
 . 5  
. 6  user.age = 25; // (*)
 . 7  
. 8  Alert (user.age); // 25
 . 9  // user but can not modify the object
 10  const = {User
 . 11    name: "John"
 12 is  };
 13 is  
14  // error (can not assign User assignment)
 15  User = {
 16    name: "Pete"
 . 17 };

　　Object replication and merging Object.assign

　　Another copy of an object variable is also equivalent to the creation of this object.

　　

1  // manipulation of the original code
 2  the let = {User
 . 3    : "John", name
 . 4    30: Age
 . 5  ;}
 . 6  
. 7  the let clone = {}; // new empty object
 . 8  
. 9  // copy all attribute values
 10  for (in the let User Key) {
 . 11    clone [Key] = User [Key];
 12 is  }
 13 is  
14  // now independent replication copy
 15  clone.name = "Pete"; // change the value
 16  
. 17 alert (user.name); // original object property value remains unchanged

　　To achieve with Object.assign

　　Syntax: Object .assign(dest [,SRC1 ,src2 ,SRC3...])

• Parameters destand src1, ..., srcN(can be many) are objects.
• This method replicates src1, ..., srcNall objects to dest. In other words, beginning from the second parameter, the properties of all objects are copied to the first parameter object, and returns dest.

  1 let user = {
  2   name: "John",
  3   age: 30
  4 };
  5 
  6 let clone = Object.assign({}, user);

  Special circumstances, the object is still the object

  . 1  the let {User =
   2    name: "John",
   . 3    sizes: {
   . 4      height: 182,
   . 5      width: 50
   . 6    }
   . 7  };
   . 8  
  . 9  the let clone = Object.assign ({}, User);
   10  
  . 11  Alert (User .sizes === clone.sizes); // true, the same object
   12 is  
  13 is  // User sizes shared objects and clone
   14  user.sizes.width ++; // here to change the value of an attribute
   15 Alert (clone.sizes. width); // 51

  In this case, the inside of the object and the object is not completely replicate, but only a copy of the value of the object, in this case, to judge again, or select a sound library j