Introduction to value type conversion in JavaScript_javascript tips
When performing operations such as -, *, /, ==, != in JavaScript, if the value types on the left and right sides of the operator are inconsistent with the expected type, JavaScript will convert the values on both sides of the operator into the expected type. Do the operation again. When the expected value type is string, JavaScript will convert the value to string; when the expected value type is number, JavaScript will convert the value to number (if it cannot be converted to a numerical value, NaN will be returned). For example:
console.log(10 "cats");//10 cats
console.log(10 * " cats");//NaN, " cats" will be converted to NaN
console.log(10 "2");//102
console.log(10 - "2");//8
console.log(10 / "2");//5
console.log(10 * "2");//20
console.log(10 * " 2");//20
console.log("10" * "2");//20
Value type conversion rules
For value conversion rules in JavaScript, please refer to Table 3-2. JavaScript type conversions in the book "JavaScript – The Definitive Guide". Some of the more noteworthy places are:
1. The result after undefined is converted to number is NaN.
2. The result of converting null to number is 0.
3. The result of converting the empty string "" into number is 0.
4. The result after converting -0 to string is "0".
5. The result of converting the empty array [] to number is 0.
6. When an array with only one number member (such as [9]) is converted into number, the result is the numer value (9).
When JavaScript converts string to number, there are two interesting rules:
1. JavaScript will delete the blank characters at the beginning and end of the string before converting it, so a string like "42" can be successfully converted into the number 42.
2. After removing the blank characters at the beginning and end, if the string still contains non-numeric characters, the string will be converted to NaN. For example: "3 m" will be converted to NaN.
Example:
console.log(10 * " 3 ");//30
console.log(10 * "3 m");//NaN, "3 m" will be converted to NaN
Value type conversion and comparison
In JavaScript, the use of the equal operator (==) involves value type conversion: if the value types on both sides of the == operator are inconsistent, then JS will convert them into consistent types before making a judgment. It should be noted that two values of different types may be equivalent after type conversion, but this does not mean that the result of using the == operator on them must be true. A simple example is undefined and false: after converting undefined to boolean type, the result happens to be false, but in fact the result of undefined==false is false.
Explicit type conversion
Using JavaScript's automatic type conversion is very convenient, but it can also easily cause problems such as code maintainability. In order to make the program code clearer and reduce ambiguity, explicit type conversion is sometimes used in JS programs:
Number("3")//3
String(false)//"false"
Boolean([])//true
In most cases, the result of explicit type conversion is consistent with the result of JS automatic type conversion; but there is a special case: when null or undefined is automatically converted to Object, JS will throw a TypeError error; but when explicit type conversion When converting null or undefined to Object, JS will return an empty object:
console.log(Object(undefined));//empty object
console.log(Object(null));//empty object
At the same time, if the type specified by explicit conversion is different from the type specified by JS automatic conversion, the results obtained will also be different. For example, the result of undefined==false mentioned before is false; if the conversion type is explicitly specified as Boolean, the result will be true:
console.log(undefined == false);//false
console.log(Boolean(undefined) == Boolean(false));//true
Use of automatic type conversion
In JS, you can use automatic conversion of value types and use operators to achieve the same effect as explicit conversion, such as:
console.log(false "");//"false"
console.log(false);//0
console.log(!!3);//true

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.

The main uses of JavaScript in web development include client interaction, form verification and asynchronous communication. 1) Dynamic content update and user interaction through DOM operations; 2) Client verification is carried out before the user submits data to improve the user experience; 3) Refreshless communication with the server is achieved through AJAX technology.


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