


The definition and usage of substring and substr in js_javascript skills
1.substring method
Definition and usage
Thesubstring method is used to extract characters between two specified subscripts in a string.
Grammar
stringObject.substring(start,stop)
Parameter Description
start Required. A nonnegative integer that specifies the position in stringObject of the first character of the substring to be extracted.
stop Optional. A nonnegative integer that is one position in the stringObject that is one more than the last character of the substring to be extracted. If this parameter is omitted, the returned substring will go to the end of the string.
Return value
A new string value containing a substring of stringObject whose content is all characters from start to stop-1, and whose length is stop minus start.
Description
The substring returned by the substring method includes the characters at start, but does not include the characters at end.
If start and end are equal, then this method returns an empty string (that is, a string of length 0).
If start is greater than end, then this method will exchange these two parameters before extracting the substring.
If start or end is negative, then it will be replaced with 0.
2.substr method
Definition and usage
Thesubstr method is used to return a substring of the specified length starting from the specified position.
Grammar
stringObject.substr(start [, length ])
Parameter Description
start Required. The starting position of the desired substring. The first character in the string has index 0.
length Optional. The number of characters that should be included in the returned substring.
Description
If length is 0 or negative, an empty string will be returned.
If this parameter is not specified, the substring will be continued to the end of stringObject.
3. Example
text/javascript">
function Demo(){
var str,str;
var s = "Hello Word";
str = s.substring (0, 3); // Get substring.
console.log (str); // ===== & gt; Hel
STR = S.Substr (0,3);
console.log (sTR); // = ====>Hel
} }

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.

JavaScript's application in the real world includes front-end and back-end development. 1) Display front-end applications by building a TODO list application, involving DOM operations and event processing. 2) Build RESTfulAPI through Node.js and Express to demonstrate back-end applications.


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