Strict mode: The JavaScript standard defined by the ECMA-262 specification has stronger restrictions on javascript. This article mainly introduces the detailed explanation of JavaScript strict mode use strict. Friends who need it can refer to
Strict mode: The JavaScript standard defined by the ECMA-262 specification has stronger restrictions on javascript.
(Non-strict mode is called "sloppy mode/sparse mode/lazy mode".)
1. Use of strict mode
Strict mode can be implemented at the script or function level. (i.e. global and local modes)
1. Global
Add "use strict" at the front of the js file
2. Local
in the function Add "use strict" internally, as follows
function fn() { "use strict"; //some code }
2. Comparison between strict mode and non-strict mode
1. In strict mode, variables cannot be deleted (delete is an unqualified identifier)). In non-strict mode, deletion failure will return false
"use strict"; var x; delete x;//报错Delete of an unqualified identifier in strict mode.(不合格的标识符)
2. In strict mode, if the function parameter has the same name, an error will be thrown; non-strict mode will not
In strict mode
function fn(a,a){ "use strict"; result=a+a; console.log(result); } fn(2,4);//Duplicate parameter name not allowed in this context(重复的参数名称在此上下文中不允许)
In non-strict mode
function fn1(a,a){ "use strict"; result=a+a; console.log(result); } fn1(2,4);//结果为8
3. Strict mode does not allow octal integers Direct quantity (below). No error will be reported in non-strict mode.
"use strict" var x=089; console.log(x);//报错:Decimals with leading zeros are not allowed in strict mode.
4. In strict mode, the arguments object is a static copy of the actual parameter list passed into the function (that is, changes in parameters are no longer tracked); in non-strict mode, The elements in the arguments object and the corresponding actual parameters are references to the same value.
In strict mode
fn(5); function fn(a){ "use strict"; a = 42; // return a==arguments[0];//返回false console.log(a);//结果为42 console.log(arguments[0]);//结果为5,严格模式下arguments[0]表示这个调用方法的第一个参数 (不再追踪 参数的变化,在函数内部,参数a被重新赋值为42,但arguments[0]仍然为5.) }
In non-strict mode
fn(5); function fn(a){ a = 42; // return a==arguments[0];//返回true console.log(a);//结果为42 console.log(arguments[0]);//结果为42(追踪参数变化) }
5. In strict mode eval and arguments are used as keywords, they cannot be assigned and used as variable declaration
"use strict"; var eval=3;//报错:Unexpected eval or arguments in strict mode var argument=6;//同样报错
6. Strict mode will limit the ability to detect the call stack, access arguments.callee, arguments.callee.caller will throw an exception
In strict mode:
"use strict"; function fn(n){ if(n==0){ return 1; }else{ return n*arguments.callee(n-1); } } console.log(fn(5));//报错
In non-strict mode:
function fn(n){//阶乘 if(n==0){ return 1; }else{ return n*arguments.callee(n-1); } } console.log(fn(5));//正常120
caller: When a function calls another function, the called function will automatically generate a caller attribute pointing to the function object that called it. If the function is not currently called, or is not called by another function,
caller is null. As follows:
In strict mode:
"use strict" function fn1() { var n1= fn1.caller; console.log(n1); } function fn2() { fn1(); } fn2();//报错
In non-strict mode:
function fn1() { var n1= fn1.caller;//此例中caller指向调用它的函数(fn2) console.log(n1); } function fn2() { fn1(); } fn2();// 结果打印出:function fn2(){ fn1( ) };
7 .Strict mode variables must be declared first and assigned directly to the variable. Global variables will not be created implicitly and with cannot be used.
In strict mode
"use strict"; with(obj){ x; }//报错:Strict mode code may not include a with statement
Explanation: When running a script, it is compiled first and then executed. Obviously at compile time, it cannot be determined at compile time which object the variables in the with statement block belong to. [Is the variable in the with statement block an attribute
of (cl) or a variable in the upper-level variable scope chain. ]. It can only be determined at execution time that (c1) is an instance of Clothes. This conflicts with strict mode, which checks whether variables are defined at compile time, so strict mode does not use the with statement.
Extension: Use of with statement (used to reference existing attributes in a specific object, but cannot be used to add attributes to the object.)
with(object instance) { //代码块 } eg:function Clothes(){ this.color="red"; this.size="m"; this.price="cheap"; } var c1=new Clothes(); with(c1){ var str="颜色:"+color+",尺寸:"+size+",价格:"+price; document.write(str); }//结果(颜色:red,尺寸:m,价格:cheap)
In strict mode, null undefined passed in to call apply remains as is and is not converted to window
The above is the detailed content of Javascript tutorial use strict strict mode detailed explanation. For more information, please follow other related articles on the PHP Chinese website!

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.

Python is more suitable for data science and automation, while JavaScript is more suitable for front-end and full-stack development. 1. Python performs well in data science and machine learning, using libraries such as NumPy and Pandas for data processing and modeling. 2. Python is concise and efficient in automation and scripting. 3. JavaScript is indispensable in front-end development and is used to build dynamic web pages and single-page applications. 4. JavaScript plays a role in back-end development through Node.js and supports full-stack development.

C and C play a vital role in the JavaScript engine, mainly used to implement interpreters and JIT compilers. 1) C is used to parse JavaScript source code and generate an abstract syntax tree. 2) C is responsible for generating and executing bytecode. 3) C implements the JIT compiler, optimizes and compiles hot-spot code at runtime, and significantly improves the execution efficiency of JavaScript.


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

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),

Dreamweaver CS6
Visual web development tools

VSCode Windows 64-bit Download
A free and powerful IDE editor launched by Microsoft

Atom editor mac version download
The most popular open source editor

Safe Exam Browser
Safe Exam Browser is a secure browser environment for taking online exams securely. This software turns any computer into a secure workstation. It controls access to any utility and prevents students from using unauthorized resources.
