Example analysis of typeof and type judgment in JavaScript

typeof

ECMAScript has 5 primitive types, namely Undefined, Null, Boolean, Number and String. We all know that we can use the typeof operator to find the type of a variable, but for reference type variables only <a href="http://www.php.cn/wiki/60.html" target="_blank">object</a> will be returned. In other words, typeof can only correctly identify basic type value variables.

var a = "abc";
console.log(typeof a); // "string"var b = 123;
console.log(typeof b); // "number"var c = true;
console.log(typeof c); // "boolean"var d = null;
console.log(typeof d); // "object"var f = undefined;
console.log(typeof f); // "undefined"var g;
console.log(typeof g); // "undefined"console.log(typeof x); // "undefined"

You may ask why the typeof operator returns "object" for null values. This was actually a bug in JavaScript's original implementation, which was then adopted by ECMAScript. Now, null is considered a placeholder for an object, thus explaining the contradiction, but technically it's still a primitive value.

The last one is strange, typeof a non-existent variable

x actually returns "object" instead of "undefined".

We are coming to the following code:

var a = function() { };
console.log(typeof a); // "function"var b = [1,2,3]; 
console.log(typeof b); // "object"var c = { };
console.log(typeof c); // "object"

Returns "object" for both arrays and objects, so a common need in our daily development is how to determine whether a variable is an array or an object.

Type judgment

Type judgment is generally to judge whether it is an array or an empty object. This is the judgment method I have used or seen every day for this requirement

Judge whether it is an array

There is an array:

var a = [1,2,3,4, 5];

Method one:

toString.call(a); // "[object Array]"

Method two:

a instanceof Array; //true

Method three:

a.constructor == Array; //true

The first method is more general and also It is the abbreviation of

Object.prototype.toString.call(a). The variables judged by

instanceof and constructor must be declared on the current page. For example, a page (parent page) has a frame, A page (subpage) is referenced in the frame, an a is declared in the subpage, and assigned to a variable of the parent page. When judging the variable, Array == object.constructor will Returnfalse;

var a = [1,2,3,4,5];
console.log(toString.call(a)); // "[object Array]"                     
console.log(a instanceof Array); //trueconsole.log(a.constructor == Array); //true

Judge whether it is an empty object

There are variables:

var obj = {};

Method 1 :

JSON.stringify(obj); // "{}"

Determine whether it is an empty brace by converting it into a JSON object

Method 2:

if(obj.id){ //如果属性id存在....}

This method is relatively simple, and most people can think of it, provided that You have to know that there is a certain attribute in the object.

Method 3:

function isEmptyObject(e) { var t; for (t in e) return !1; return !0 } //trueisEmptyObject(obj); //falseisEmptyObject({ "a":1, "b":2});

This method is the implementation of jQuery’s isEmptyObject() method.

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