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typeof
ECMAScript has 5 primitive types, namely Undefined, Null, Boolean, Number and String. We all know that you can use the typeof operator to find the type of a variable, but for reference type variables, it will only return object, which means that 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 the original implementation of JavaScript that 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 daily 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, which 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 (child page) is referenced in the frame, and a page is declared in the child page. a, and assign it to a variable of the parent page. When judging the variable, Array == object.constructor will return false;
var a = [1,2,3,4,5]; console.log(toString.call(a)); // "[object Array]" console.log(a instanceof Array); //true console.log(a.constructor == Array); //true
Judge whether it is an empty object
There are variables: var obj = {};
Method 1:
JSON.stringify(obj); // "{}" is converted into a JSON object to determine whether it is an empty brace
Method 2:
if(obj.id){ //If the attribute id exists...} This method is relatively simple and most people can think of it. The premise is that there is a certain object in the object. Attributes.
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.
Related recommendations:
Introduction to the use of typeof in JavaScript
Detailed examples of comparative usage of typeof and instanceof in JavaScript
Summary of the usage of typeof in js
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