Detailed analysis of JavaScript variable types_javascript skills
Variable type
typeof,instanceof
var obj = null; console.info(typeof obj); //Object var arr = []; console.info(arr instanceof Object); //true console.info(arr instanceof Array); //true
Wapper Object of primitive data type
Data type conversion
console.log(parseInt("34", 10)); //34 console.log(parseInt("34s5b", 10)); //34 console.log(parseInt("s", 10)); //NaN console.log(parseInt(3.14, 10)); //3
//number var value = 100; //string value = "qiu"; //object value = [1, 'two', 3];
"="Various patterns of numbers:
because === Strict judgment and other
var x = 42; var y = "42"; console.log(x == y) //true; console.log(x === y) //false
Udefine: means the variable is undefined and does not have a valid value (there is no variable yet, so how can we talk about the value)
null: nothing, a variable does not refer to any object. null is an object, type object (has variables, but does not reference values)
var obj = null; if (obj === null) { alert("obj === null"); //此句将执行 } else { alert("obj!=null"); } alert(typeof obj); //object
var myVar; //true console.log(typeof myVar === "undefined"); console.log(myVar === undefined); var myVar2 = null; console.log(typeof myVar2); //object //true; console.log(myVar2 == null); console.log(myVar2 === null); //true console.info(myVar == myVar2); //undefine == null; 是true //false console.info(myVar === myVar2); //undefine === null; 是false
undefined, null,NaN,"",0
Except for these values, other values are true;
Operator: ! ! with ||
!! Convert the following expression into a boolean value and return true or false
!! "qiu"
var ns = ns || {}
If ns is undefined, return {}, otherwise return ns
Note: You must use var to define variables, otherwise you will get stuck! If you don’t write var, it will become a global variable
The above is the entire content of this article, I hope you all like it.

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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