Javascript beginners Article 4 js objects and arrays page 1/2

It's hard. Prepare yourself ...... Take a deep breath...

1, for/in:
A Method for Traversing (enumerative) object attributes can loop through attributes we do not know beforehand.
It can enumerate all user-defined attributes, but cannot enumerate some predefined attributes and methods.
Attributes that cannot be enumerated are generally inherited attributes.

Delete the attributes of an object:
Delete book. width;
If you delete an attribute from an object, for/in does not enumerate the attribute, and width in book does not detect the attribute.

Another important purpose of for/in is to use it with the associated array: (if you forget the definition of the associated array, you can refer to the previous chapter .)
For (stoct in port ){
Value + = get_value (stoct) * port [stoct];
}

2. Common Object Attributes and methods:
1): constructor attributes:
Each object has this attribute, which references the constructor that initializes this object.
For example:
Var d = new Date (); // use Date () to create an object d;
D. constructor = Date; // true // attribute d. constructor references Date;

This attribute helps determine the type of an object;
For example:
We want to determine whether the type of a value is Date:
If (typeof o = "object") & (o. constructor = Date )){
// First check whether it is an object and then check whether Date is referenced
}

The above code can also be written as follows:
If (typeof o = "object") & (o instanceof Date )){
// The instanceof operator is used to detect the value of the o. constructor attribute.
}

3. toStirng () and toLocaleString () methods:
1): toLocaleStirng () returns a localized string of the object.
ToString and toLocaleString are returned in the same way, but in the subclass, there is a difference:
For example:
Array, Date, and Number define the toLocaleString () method for returning localized values.

4. hasOwnProperty () and propertyIsEnumerable () methods:
1): hasOwnProperty
Var a = {x: 1, y: 2 };
Var k = a. hasOwnProperty ("x ");
Alert (k) // true
Alert (Math. hasOwnProperty ("z"); // false
Alert (Math. hasOwnProperty ("cos"); // true
Note: Math, cos (): calculates in radians and returns the cosine of the specified angle.
PropertyIsEnumerable () is the same as the returned result with hasOwnProperty;

4. isPrototypeOf () method:
If the object to which the method belongs is the prototype object of the parameter.
Var a = {x: 1, y: 2 };
Var k1 = Object. prototype. isPrototypeOf (a); // o. constructor = Object
Var k2 = Object. prototype. isPrototypeOf (Function); // Function. constructor = Object
Alert (k1) // true
Alert (k2) // true

5, array:
1) Create an array:
Direct array quantity:
Var es = [];
Complex point var es = [[1, {x: 1, y: 2}], [2, {x: 3, y: 4}];
Another method is to use Array () to construct a function:
V1: No parameter:
Var a = new Array ();
Empty array, equal to var a =;

V2: multiple parameters:
Var a = new Array (1, 2, 3, "tt"); // you can see that the direct definition is simpler.

V3: 1 numeric parameter:
Var a = new Array (3 );
An array with three elements. The value of each element is undefined;

6. subscript (INDEX) of the array ):
Size: 0 <= subscript <2 to the power of 32-1;
If it is out of the range, js will convert it into a string as the name of the object property;
Instead of as the subscript of the array;
For example:
A [-1.2] = "test"; // equivalent to a ["-1.2"] = "test ";
// Code explanation: Create an attribute named "-1.2" instead of defining a new array element.