JavaScript Data Types and How to Judge

Keywords: Javascript

First: How many data structures are there in ES6? What are the specific ones?
Generally speaking, as of ES6, there are 8 kinds of data decomposition: 7 kinds of original data decomposition + 1 kind of complex data decomposition.
Seven kinds of original data deconstruction:


One kind of complex data deconstruction:


2. How to judge the type of a data
There are many ways to determine the data type of a variable, but some methods are not applicable to any data type.

1: typeof
typeof can be used to determine all basic data types, returning a string whose value is in lowercase form of the type name.

//Six types of raw data
console.log(typeof 'a');//"string"
console.log(typeof 1);//"number"
console.log(typeof true);//"boolean"
console.log(typeof Symbol());//"symbol"
console.log(typeof undefined);//"undefined"
console.log(typeof null);//"object"
//object type
console.log(typeof {});//"object"
console.log(typeof []);//"object"
console.log(typeof new Map());//"object"
console.log(typeof new Set());//"object"
//function type
console.log(typeof function () {});//"function"

From the above code, we can see that there are three types of data returned by typeof, which may deserve our special attention:

1: null is the original data type, but typeof(null) gets'object'
2: Array, Set, Map data, typeof results are'object'
3: function is the core of the whole JavaScript language. Using typeof for a function, we get'function'

2: instanceOf

Grammar: object instance of constructor

InstanceOf is used to determine whether an object is an instance of a constructor, and the return value is true or false. The difference between instanceOf and typeof is that instanceOf is only used for object type data:

let fun = new Function();
function Apple() {}
let apple = new Apple();

console.log(fun instanceof Function);//true
console.log(apple instanceof Apple);//true
console.log(apple instanceof Function);//false

console.log([] instanceof Array);//true
console.log([] instanceof Object);//true

console.log(new Map() instanceof Map);//true
console.log(new Map() instanceof Object);//true

console.log({} instanceof Object);//true

In particular, Array, Set, Map have their own constructors, but Object is the root constructor, so use instanceOf Object for them, and of course return true.

3: constructor

Grammar: o.constructor

The constructor returns the constructor of an object, not a string. Conceptually, it seems to be similar to instanceOf, but after the actual operation, the results are different. Let's first look at the code:

let fun = new Function();
function Apple() {}
let apple = new Apple();

console.log(fun.constructor === Function);//true
console.log(apple.constructor === Apple);//true
console.log(apple.constructor === Function);//false

console.log([].constructor === Array);//true
console.log([].constructor === Object);//false

console.log(new Map().constructor === Map);//true
console.log(new Map().constructor === Object);//false

console.log({}.constructor === Object);//true

When we get the constructor for Array and Map, we get the most specific constructor, so their constructor is not equal to Object.

4: prototype
Prototype is a prototype object. All instantiated objects inherit the properties and methods on the prototype object.
All objects, such as {}, inherit attributes and methods on Object.prototype; Array, Set, Map are also inherited from Object, so we can also use prototype to determine the type of object:

let fun = new Function();
function Apple() {}
let apple = new Apple();

console.log(; //"[object Function]"
console.log(; //"[object Object]"
console.log([]));//"[object Array]"
console.log( Map()));//"[object Map]"
console.log({}));//"[object Object]"

Posted by xubi on Sun, 08 Sep 2019 03:16:29 -0700