JavaScript specifies several languages

JavaScript is a high-level programming language that was first designed and developed by Netscape programmer Brendan Eich in 10 days. It is widely used in web development to add interactivity and dynamic effects to websites. JavaScript specifies several languages, and this article will explore this issue in depth.

JavaScript is a dynamic language that is interpreted and executed. Because of its openness and ease of learning, it has become an essential skill in web application development. In the world of JavaScript, there are many different language types. The following are several languages ​​​​specified by JavaScript:

1. Basic data types

JavaScript specifies 6 basic data types: Undefined, Null, Boolean, Number, String and Symbol . Symbol is a new data type in ES6, which can be used as a unique identifier. Primitive data types are also called primitive types because they are immutable and all operations return new values.

2. Object type

In JavaScript, except for basic data types, all other types belong to object types. Object types can be divided into three categories: built-in objects, host objects and custom objects. Built-in objects include Object, Array, Date, RegExp, etc., which are provided by the JavaScript engine. Host objects are objects provided by the web browser or Node.js, such as window, document, XMLHttpRequest, etc. Custom objects are objects defined by developers themselves.

3. Function type

Function type is also an important type in JavaScript. Functions in JavaScript are actually special objects that can be passed and assigned like objects. Functions can be defined through function declarations, function expressions, arrow functions, and can be used in various scenarios such as callback functions, closures, and modules.

4. Type conversion

Data type conversion in JavaScript is very flexible. In different scenarios, data types are converted automatically or forcibly. Among them, implicit type conversion is the most common type conversion method. For example, when adding a string, it will be automatically converted to a numeric type.

5. Execution context

Execution context is a very important concept in JavaScript. When JavaScript code is executed, an execution context is created to store information such as variables, functions, and this of the current code. The execution context forms a stack, called the execution context stack. When the function is called, the new execution context will be pushed onto the top of the stack. When the function completes execution, its execution context will be popped.

In JavaScript, execution context includes three different types: global execution context, function execution context and eval execution context. The global execution context is the top-level execution context in all code, while the function execution context and eval execution context are created when the code is executed in a function or eval function. Each execution context has a variable environment (Variable Environment) and a lexical environment (Lexical Environment), which are used to store variables, functions, and other information.

6. Closure

Closure is an important concept in JavaScript and a common programming technique. A closure is a function that can access free variables even after the parent function has completed execution. Closures can be used in many scenarios, such as modular development, asynchronous programming, etc.

In short, JavaScript specifies different language types, including basic data types, object types, function types, execution context, type conversions and closures. These concepts are important for a deep understanding of the JavaScript language itself, as well as web development. Mastering these knowledge points can allow us to better understand the working principle of JavaScript code, so as to more efficiently develop web applications with strong functionality and stable performance.

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