On the _javascript techniques of JS data type judgment and array judgment

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1. JS six major data types

Number: numbers, integers, floating points, and so on,

String: Single or double quotes to illustrate,

Boolean: Returns True and false, these values do not necessarily correspond to 1 and 0

Object: Objects that can be created by executing the new operator followed by the name of the object type to be created.

Null: Only one worthy data type, logically, a null value represents an empty object pointer.

Undefined: Undefined, the value of the variable is undefined when the variable is declared with Var but not initialized.

2. typeof of data type judgment

typeof can resolve most data types, and its return value is a string that describes the type of operand.

Determines whether the variable num is a numeric type
if (typeof num== ' number ') {return
  true;
}

return Result:

var a= "hling"; Console.log (a); String
var a=1; Console.log (a);//number
var a=false; Console.log (a);//boolean
var A; Console.log ( typeof a);  Undfined

var a = null; Console.log (typeof a);//object
var a = document; Console.log (typeof a);//object
var a = []; Console.log (a); Object

var a = function () {}; Console.log (typeof a)//function can determine the type of a 
function in addition to the data type

In addition to the four types of string, number, Boolean, and Undefined, null, object, array return are object types!!!

Functions are returned for function types, such as typeof (Date), typeof (Eval), and so on.

3. js method of judging array type

1) instanceof

Instanceof is used to determine whether a variable is an instance of an object, a three-mesh expression. This operator has something to do with JavaScript-oriented objects, so you need to understand the object-oriented in JavaScript first. Because this operator is the prototype object that detects whether the object's prototype chain points to the constructor.

A instanceof B?alert ("true"): Alert ("false") 
//Note that the B value is the type of data you want to judge, is not a string, such as an array

Example:

var arr = [1,2,3,1]; 

2) constructor

Definition in the Consortium definition: constructor property returns a reference to the array function that created this object

var arr = []; 
Arr instanceof Array; True 
Arr.constructor = = Array;//true

The methods to judge various types are:

Console.log ("string". Constructor = string);
Console.log ((123). constructor = number);
Console.log (False.constructor = = Boolean);
Console.log ([].constructor = = Array);
Console.log ({}.constructor = = Object);

Common methods:

function IsArray (object) {return
  object && typeof object=== ' object ' &&
      Array = = Object.constructor;
}

3) characteristic judgment

Object.isarray () to determine whether a value is accurately detected as an array. ie9+, Firefox 4+, Safari 5+, Opera 10.5+, and chrome all implement this approach. However, the version prior to IE8 is not supported.

function IsArray (object) {return
  object && typeof object=== ' object ' &&  
      typeof object.length== = ' number ' && 
      typeof object.splice=== ' function ' &&  
       //Determine whether the length property is enumerable for the array will get false 
      ! Object.propertyisenumerable (' length '));

4) Object.prototype.toString.call

Object.prototype.toString.call (value) = = ' [Object Array] '

The above discussion JS data type judgment and the array judgment is small arranges to share to everybody's content, hoped can give everybody a reference, also hoped that everybody supports the cloud habitat community.