Example analysis of typeof and type judgment in JavaScript-JS Tutorial-php.cn

Example analysis of typeof and type judgment in JavaScript

黄舟

Release： 2017-10-27 09:20:34

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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"

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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"

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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]"

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Method two:

a instanceof Array; //true

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Method three:

a.constructor == Array; //true

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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

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Judge whether it is an empty object

There are variables:

var obj = {};

Method 1 :

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

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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....}

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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});

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

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