Several ways to determine data type in js

There are several methods to determine the data type in js: typeof, instanceof, constructor, prototype, $.type()/jquery.type(). Next, we will mainly compare the similarities and differences of these methods.

Let’s give a few examples:

var a = "iamstring.";
var b = 222;
var c= [1,2,3];
var d = new Date();
var e = function(){alert(111);};
var f = function(){this.name="22";};　　

1. The most common judgment method: typeof

alert(typeof a)  ------------> string
alert(typeof b)  ------------> number
alert(typeof c)  ------------> object
alert(typeof d)  ------------> object
alert(typeof e)  ------------> function
alert(typeof f)  ------------> function

where typeof returns The types are all in string form, please note, for example:

alert(typeof a == "string") -------------> true
alert(typeof a == String) ---------------> false

In addition, typeof can determine the type of function; it is more convenient when determining objects other than Object type .

2. Method to determine known object type: instanceof

alert(c instanceof Array) ---------------> true
alert(d instanceof Date)
alert(f instanceof Function) ------------> true
alert(f instanceof function) ------------> false

Note: instanceof must be followed by the object type and size You can’t go wrong in writing. This method is suitable for some conditional selection or branching.

3. Judge based on the constructor of the object: constructor

alert(c.constructor === Array) ----------> true
alert(d .constructor === Date) -----------> true
alert(e.constructor === Function) -------> true

Note: constructor will make an error when inheriting a class

eg：

function A(){};
function B(){};
A.prototype = new B(); //A继承自B
var aObj = new A();
alert(aobj.constructor === B) -----------> true;
alert(aobj.constructor === A) -----------> false;

This problem will not occur with the instanceof method. Both direct and indirect inheritance of objects will report true:

alert(aobj instanceof B) ----------------> true;
alert(aobj instanceof B) ----------------> true;

Getting back to the subject, to solve the problem of constructor is usually to let the object's constructor point to itself manually:

aobj.constructor = A; //将自己的类赋值给对象的constructor属性
alert(aobj.constructor === A) -----------> true;
alert(aobj.constructor === B) -----------> false; //基类不会报true了;

4. Common but cumbersome method : prototype

alert(Object.prototype.toString.call(a) === ‘[object String]') -------> true;
alert(Object.prototype.toString.call(b) === ‘[object Number]') -------> true;
alert(Object.prototype.toString.call(c) === ‘[object Array]') -------> true;
alert(Object.prototype.toString.call(d) === ‘[object Date]') -------> true;
alert(Object.prototype.toString.call(e) === ‘[object Function]') -------> true;
alert(Object.prototype.toString.call(f) === ‘[object Function]') -------> true;

You can’t write the wrong capitalization, which is more troublesome, but it’s more general.

5. The invincible and omnipotent method: jquery.type()

If the object is undefined or null, the corresponding "undefined" or "null" will be returned.

jQuery.type( undefined ) === "undefined"
jQuery.type() === "undefined"
jQuery.type( window.notDefined ) === "undefined"
jQuery.type( null ) === "null"

If the object has an internal [[Class]] that is the same as a browser's built-in object's [[Class]], we return the corresponding [[Class]] name. (More details on this technique.)

jQuery.type( true ) === "boolean"
jQuery.type( 3 ) === "number"
jQuery.type( "test" ) === "string"
jQuery.type( function(){} ) === "function"
jQuery.type( [] ) === "array"
jQuery.type( new Date() ) === "date"
jQuery.type( new Error() ) === "error" // as of jQuery 1.9
jQuery.type( /test/ ) === "regexp"

Everything else will return its type "object".

Usually, you can use typeof to judge. If you encounter a situation where the Object type is known, you can use the instanceof or constructor method. If you have no choice, use the $.type() method.

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