js: Several ways to judge the data type

Reference article: Summary of several methods for judging object types in JavaScript

According to the data object to be judged, choose the appropriate method

1. Original data type

6种：number、string、boolean、undefined、null、symbol

1. typeof

let str = 'hello';
console.log(typeof str);	// string

【special! ! 】The type of null cannot be judged by typeof, instanceof, and constructor.
let s = null; console.log(typeof s) //Object

The following are two methods for judging whether it is a null type:
console.log(s === null)
console.log(Object.prototype.toString.call(s) === '[object Null]');

2. For some reference data types: object, function, typeof can also be used

let obj = {
    
    }
console.log(typeof obj);	// object

function func(){
    
    
	return 'this is a function';
}
console.log(typeof func);	// function

3. For other reference data types: Array, Date, RegExp

typeof has limitations: For some of the reference data types listed in the third case, using typeof can only display objects, not specific Arrays. [Cannot be used to distinguish: one object type from another object type]

let arr = [1,2,3,4];
console.log(typeof arr); // object，此时就不能typeof了

two, instanceof

let arr = [1,2,3,4];
console.log(arr instanceof Array);	// true

The instanceof operator requires that its left operand is an object, and its right operand is the name of the object's class or a constructor. The instanceof operator returns true if object is an instance of a class or constructor. Returns false if object is not an instance of the specified class or function, or if object is null.

let str = 'hello';
console.log(str instanceof String); // false

Three, constructor attribute

let arr = [1,2,3,4];
console.log(arr.constructor);	// ƒ Array() { [native code] }，引用了初始化该对象的构造函数
console.log(arr.constructor === Array);	// true

let date = new Date();
console.log(typeof date);	// object
console.log(date.constructor);	// Date
let reg = new RegExp("/^/d*$/") ; 	
console.log(typeof reg);	// object
console.log(reg.constructor);	// RegExp

Supplement: What exactly is a JS prototype

The constructor attribute in the prototype object points to the constructor

[Thinking]: Is there any connection between the third and fourth methods?
Is arr a prototype object or an instance object?

4. For cross-framework

Let's understand the following passage by yourself, I don't understand it very well

四、Object.prototype.toString()

Reference: Usage of call(), apply(), bind() in JavaScript

call() is used to change the point of this, and apply and bind can be used in the same way

[Note]: bind returns a new function, which needs to be called again

let arr = [1,2,3,4];
console.log(Object.prototype.toString.call(arr));	// [object Array]
console.log(Object.prototype.toString.call(arr) === "[object Array]");

console.log(Object.prototype.toString.apply(arr));	
console.log(Object.prototype.toString.apply(arr) === "[object Array]");
console.log(Object.prototype.toString.bind(arr));	// ƒ toString() { [native code] }
console.log(Object.prototype.toString.bind(arr)());	//[object Array]
console.log(Object.prototype.toString.bind(arr)() === "[object Array]");

at last

Note : In order to describe intuitively, there is a situation where a variable is declared multiple times in the text. In the code, a variable declared with let can only be declared once, otherwise an error will be reported, but the variable value can be modified later