What are typeof and instanceof in js? What's the difference?
This article mainly introduces to you the difference between typeof and instanceof in js.
First of all, everyone should briefly understand what typeof is?
typeof is a unary operation, placed before an operand, and the operand can be of any type. Its return value is a string describing the type of the operand. typeof can be used to detect the data type of a given variable.
What is instanceof?
The instanceof operator is used to determine whether the object pointed to by the prototype attribute of a constructor exists on the prototype chain of another object to be detected. Generally speaking, using instanceof is to determine whether an instance belongs to a certain type.
The similarities between typeof and instanceof in js:
Typeof and instanceof in JavaScript are often used to determine whether a variable is empty or what type it is.
Differences:
1. The definition and usage of typeof:
The return value is a string , used to describe the data type of the variable.
Specific usage details:
1. typeof generally can only return the following results:
'undefined': This value is undefined.
'boolean': This value is a Boolean value.
'string' : This value is a string.
'number' : This value is a numeric value.
'object': This value is an object or null.
'function' : This value is a function.
2. Use typeof to get whether a variable exists, such as
if(typeof a!="undefined"){alert("ok")}
instead of using if(a) because if a does not exist (not declared), an error will occur.
3. For special objects such as Array and Null, typeof always returns object. This is the limitation of typeof.
2. Instanceof definition and usage:
Instanceof definition and usage: instanceof is used to determine whether a variable belongs to an instance of an object. It can also be used to determine whether the prototype attribute of a certain constructor exists on the prototype chain of another object to be detected.
Example:
a instanceof b?alert("true"):alert("false"); //a是b的实例?真:假
var a=new Array();alert(a instanceof Array);
will return
true, and
alert(a instanceof Object)
will also return true;
This is because Array is an object subcategory.
Another example:
function test(){};var a=new test();alert(a instanceof test)
will return object.
Test:
var a=new Array();if (a instanceof Object) alert('Y');else alert('N');
got 'Y', and
if (window instanceof Object) alert('Y');else alert('N');
got 'N'.
So, the object tested by instanceof here refers to the object in js syntax, not the dom model object.
There will be some differences when using typeof:
alert(typeof(window))
You will get object.
This article is about the difference between typeof and instanceof in js. I hope it will be helpful to friends in need!
If you want to know more front-end knowledge points, you can follow the PHP Chinese website JavaScript Video Tutorial, Bootstrap Video Tutorial and other related tutorials. Welcome everyone to refer to and learn!
The above is the detailed content of What are typeof and instanceof in js? What's the difference?. For more information, please follow other related articles on the PHP Chinese website!

JavaScript originated in 1995 and was created by Brandon Ike, and realized the language into C. 1.C language provides high performance and system-level programming capabilities for JavaScript. 2. JavaScript's memory management and performance optimization rely on C language. 3. The cross-platform feature of C language helps JavaScript run efficiently on different operating systems.

JavaScript runs in browsers and Node.js environments and relies on the JavaScript engine to parse and execute code. 1) Generate abstract syntax tree (AST) in the parsing stage; 2) convert AST into bytecode or machine code in the compilation stage; 3) execute the compiled code in the execution stage.

The future trends of Python and JavaScript include: 1. Python will consolidate its position in the fields of scientific computing and AI, 2. JavaScript will promote the development of web technology, 3. Cross-platform development will become a hot topic, and 4. Performance optimization will be the focus. Both will continue to expand application scenarios in their respective fields and make more breakthroughs in performance.

Both Python and JavaScript's choices in development environments are important. 1) Python's development environment includes PyCharm, JupyterNotebook and Anaconda, which are suitable for data science and rapid prototyping. 2) The development environment of JavaScript includes Node.js, VSCode and Webpack, which are suitable for front-end and back-end development. Choosing the right tools according to project needs can improve development efficiency and project success rate.

Yes, the engine core of JavaScript is written in C. 1) The C language provides efficient performance and underlying control, which is suitable for the development of JavaScript engine. 2) Taking the V8 engine as an example, its core is written in C, combining the efficiency and object-oriented characteristics of C. 3) The working principle of the JavaScript engine includes parsing, compiling and execution, and the C language plays a key role in these processes.

JavaScript is at the heart of modern websites because it enhances the interactivity and dynamicity of web pages. 1) It allows to change content without refreshing the page, 2) manipulate web pages through DOMAPI, 3) support complex interactive effects such as animation and drag-and-drop, 4) optimize performance and best practices to improve user experience.

C and JavaScript achieve interoperability through WebAssembly. 1) C code is compiled into WebAssembly module and introduced into JavaScript environment to enhance computing power. 2) In game development, C handles physics engines and graphics rendering, and JavaScript is responsible for game logic and user interface.

JavaScript is widely used in websites, mobile applications, desktop applications and server-side programming. 1) In website development, JavaScript operates DOM together with HTML and CSS to achieve dynamic effects and supports frameworks such as jQuery and React. 2) Through ReactNative and Ionic, JavaScript is used to develop cross-platform mobile applications. 3) The Electron framework enables JavaScript to build desktop applications. 4) Node.js allows JavaScript to run on the server side and supports high concurrent requests.


Hot AI Tools

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

Undress AI Tool
Undress images for free

Clothoff.io
AI clothes remover

Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Article

Hot Tools

SAP NetWeaver Server Adapter for Eclipse
Integrate Eclipse with SAP NetWeaver application server.

Zend Studio 13.0.1
Powerful PHP integrated development environment

Atom editor mac version download
The most popular open source editor

ZendStudio 13.5.1 Mac
Powerful PHP integrated development environment

mPDF
mPDF is a PHP library that can generate PDF files from UTF-8 encoded HTML. The original author, Ian Back, wrote mPDF to output PDF files "on the fly" from his website and handle different languages. It is slower than original scripts like HTML2FPDF and produces larger files when using Unicode fonts, but supports CSS styles etc. and has a lot of enhancements. Supports almost all languages, including RTL (Arabic and Hebrew) and CJK (Chinese, Japanese and Korean). Supports nested block-level elements (such as P, DIV),
