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.
The above is the entire content of this article. I hope that the content of this article can bring some help to everyone's study or work. I also hope to support the PHP Chinese website!
For more js related articles on several methods of determining data types, please pay attention to the PHP Chinese website!
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