How does Javascript determine data type and array type?

Such a basic thing really shouldn’t be recorded anymore, but let’s review the past and learn something new~ Let’s start with the data types

JS six major data types: number, string, object, Boolean, null, undefined

string: Described by single quotes or double quotes, such as "string"

number: Integers and floating point numbers are all called numbers, you know~

Boolean: They are true and false

undefined: Undefined means that you create a variable but do not assign a value to it~

null: Hence the name Sijiu, null means nothing and means nothing

object: It’s hard for me to explain this. That is, types other than the above five types

--------------------The ones above are all floating clouds, and the ones below are the gods--- --------------------------

Data type judgment typeof

typeof can solve most of the problems Data type judgment is a unary operation. It is placed before an operation value. The return value is a string. The string describes the type of the operand. Therefore, to judge whether a certain one is of String type, you can directly if(typeof(your value) == "string"){}

The following are the return results of various data types:

var a="string"; console.log(a); //string
var a=1; console.log(a); //number
var a=false; console.log(a); //boolean
var a; console.log(typeof a); //undfined
 
var a = null; console.log(typeof a); //object
var a = document; console.log(typeof a); //object
var a = []; console.log(a); //object
 
var a = function(){}; console.log(typeof a) //function 除了可以判断数据类型还可以判断function类型

It is obvious that, in addition to the first four types, null, object , Arrays return object types;

For function types, function is returned, such as typeof(Date), typeof(eval), etc.

Then this can lead to another very popular problem whose solutions are already common. How to determine whether the data is an array type?

------------------------------------------Actually, this is me The purpose, eh~------------------------------------------------ -

js method to determine array type

One of the methods instanceof

instance, as the name suggests, instance, example, so instanceof is used to determine whether a variable is a certain An instance of an object is a ternary arithmetic expression---the most substantial difference from typeof

a instanceof b?alert("true"):alert("false") //Note that the b value is you The data type you want to determine is not a string, such as Array

For example:

var a=[];
console.log(a instanceof Array) //返回true

Method 2 constructor

Definition in the W3C definition: The constructor attribute returns a reference to the array function that created this object.

is to return the constructor corresponding to the object. The definition is not consistent with instanceof, but the effect is the same

For example: (a instanceof Array) //Is a an instance of Array? true or false

　 (a.constructor == Array) // Is the constructor corresponding to an instance an Array? true or false

For example:

function employee(name,job,born){
 this.name=name;
 this.job=job;
 this.born=born;
}
 
var bill=new employee("Bill Gates","Engineer",1985);
console.log(bill.constructor); //输出function employee(name, jobtitle, born){this.name = name; this.jobtitle = job; this.born = born;}

Then the way to judge the various types is:

console.log([].constructor == Array);
console.log({}.constructor == Object);
console.log("string".constructor == String);
console.log((123).constructor == Number);
console.log(true.constructor == Boolean);

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More rigorous and universal method:

function isArray(object){
 return object && typeof object==='object' &&
   Array == object.constructor;
}

! ! Note:

When using instanceof and construcor, the array being judged must be declared on the current page! For example, a page (parent page) has a frame, and a page (child page) is referenced in the frame. An array is declared in the child page and assigned to a variable of the parent page. At this time, the variable is judged, Array == object.constructor; will return false;

Cause:

1. Array is reference data, and during the transfer process, only the reference address is transferred.
2. The address referenced by the Array native object of each page is different. The corresponding constructor of the array declared in the sub-page is the Array object of the sub-page; the parent page makes the judgment and uses the Array It is not equal to the Array of subpages; remember, otherwise it will be difficult to track the problem!

Method Three: Characteristic Judgment Method

The above methods all have certain flaws, but we must believe that the wisdom of the people is omnipotent. We can judge the array based on some of its characteristics. The type

function isArray(object){
 return object && typeof object==='object' &&
   typeof object.length==='number' &&
   typeof object.splice==='function' &&
    //判断length属性是否是可枚举的 对于数组 将得到false
   !(object.propertyIsEnumerable('length'));
}

has length and splice and is not necessarily an array, because attributes can be added to the object, but the length attribute cannot be enumerated, which is the most important judgment factor.

ps: Popularize the propertyIsEnumerable method here:

object. propertyIsEnumerable(proName)

Judge whether the specified property is enumerable

Remarks: If proName Exists in object and can be exhausted using a For...In loop, then the propertyIsEnumerable property returns true. The propertyIsEnumerable property returns false if the object does not have the specified property or if the specified property is not enumerable.

propertyIsEnumerable property does not consider objects in the prototype chain.

Example:

var a = new Array("apple", "banana", "cactus");
document.write(a.propertyIsEnumerable(1));

The fourth method is the simplest method

function isArray(o) {
 return Object.prototype.toString.call(o) === ‘[object Array]‘;
}

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