JS four formulas for judging data types

1.Js Determine the type of data 4 Kind of ⽅ type

Js data type es5 6 Type of data ：string、number、boolean、object(Data、function、Array)、null、undefined
Es6 Added in ⼀ Type of data ：symbol： Express independence ⼀⽆⼆ Value , most ⼤ Of ⽤ Law is ⽤ To define the object ⼀ Property name ,⽆ On inequality at all times

var s1 = Symbol("symbol");

1.typeof

• For basic types , except null outside , Can return the correct result .
• For citation ⽤ type , except function outside ,⼀ Law return object type .
• about null , return object type .
• about function return function type .
  Typeof NaN = number
  Typeof null = object
  Typeof object = function Object/Array/String/Number/Boolean/Date/RegExp All constructors
  Typeof {} = object
  typeof console.log = ‘function’ The function itself
  typeof console.log() = ‘undefined’ shipment ⾏ function , Default return undefined
  2.instanceof
  test A Whether it is B Example ,⽐ a A._proto Whether and B.prototype equal
  ,instanceof Can only ⽤ To determine whether the two objects belong to the instance relationship , ⽽ Can't judge ⼀ Which type of object instances belong to .
  [] instanceof Array
  {} instanceof Object

instanceof (A,B) = {
    var L = A.__proto__;
    var R = B.prototype;
    if(L === R) {
        // A The internal properties of  __proto__  Point to  B  Prototype object 
        return true;
    }
    return false;
}

3.constructor

1. null and undefined yes ⽆ Effective object , So there won't be constructor There is , These two types of data need to be passed through other ⽅ To judge .
   ‘’.constructor === ‘String’
   New Number(1).constructor === ‘Number’
   New Function().constructor === ‘function’
   4.toString
   Object The prototype of the ⽅ Law , Returns the... Of the current object [[Class]]
   Object.prototype.toString.call(‘’) // [[Object String]]
   Object.prototype.toString.call(new Error()) ; // [object Error]
   Object.prototype.toString.call(document) ; // [object HTMLDocument]
   Object.prototype.toString.call(window) ; //[object global] window Global object global Introduction of ⽤