Example Analysis of Type judgment in javascript

Editor to share with you the javascript type judgment example analysis, I believe that most people do not know much about it, so share this article for your reference, I hope you will learn a lot after reading this article, let's go to know it!

There are two types of data in Javascript:

Simple data types: Undefined, NULL, Boolean, Number, String

Complex data type: Object

Next let's take a look at how to do data type discrimination.

First, let's take a look at typeof.

TypeResultUndefined "undefined" Null "object" (see below) Boolean "boolean" Number "String" string "Symbol (new in ECMAScript 2015)" symbol "Host object (provided by the JS environment) Implementation-dependentFunction object (implements [[Call]] in ECMA-262 terms)" function "Any other object" object "

Have some code demo.

Let a = undefined;typeof a "undefined" let b = false;typeof b "boolean" let c = 12 position typeof c "number" let d ='12 let type of d "string" let f = function () {}; typeof f "function"

Next let's take a look at those strange phenomena.

Let str = new String ('abc'); typeof str "object" let num = new Number (12); typeof num "object" var func = new Function (); typeof func; "function" typeof null "object"

Variables created using the constructor return a "object" result using typeof judgment, with the exception of the Function function, where the variable typeof created by it returns "function"

Then let's talk about typeof null = = "object". I believe that the front-end development partners all know this result, But why? This is actually a bug that existed in the first version of javascript. For historical reasons, you can take a look at this article The history of typeof null.

About how to judge an array

Let arr = [1,2,3]; typeof arr "object"

The above result should be no stranger to everyone, so how to correctly judge the array type?

1 、 instanceof

Arr instanceof Array / / true

2 、 isArray

Array.isArray (arr) / / true

3 、 constructor.name

Arr.constructor.name / / "Array"

The third usage should be used by relatively few people, and many front-end friends have never used it. For the complex type Object, each instance of it has a constructor property.

Instanceof vs isArray

When detecting Array instances, Array.isArray is better than instanceof because Array.isArray can detect iframes.

Var iframe = document.createElement ('iframe'); document.body.appendChild (iframe); xArray = new xArray [window.frames.length-1] .Array; Array = new xArray; / Correctly checking for ArrayArray.isArray (arr); / / true// Considered harmful, because doesn't work though iframesarr instanceof Array; / / false

This code is from MDN copy. In addition to the following results, it is found that the third method constructor.name can also be correctly judged.

Arr.constructor.name / / "Array"

About NaN

Use isNaN to determine NaN.

IsNaN (1) / / true

We know that the result of NaN = = NaN is false, so how do we judge two NaN variables?

To compare the two NaN variables, use es6's Object.is ().

Let nan1 = NaNlet nan2 = NaNObject.is (nan1, nan2) true is all the content of the article "sample Analysis of Type determination in javascript". Thank you for reading! I believe we all have a certain understanding, hope to share the content to help you, if you want to learn more knowledge, welcome to follow the industry information channel!