javascript script execution process

JavaScript (JS for short) is a high-level programming language commonly used for writing web applications. When writing JavaScript, it is crucial to understand its script execution process. This article will introduce the JavaScript script execution process in detail.

1. Parsing JavaScript code

Before the JavaScript code is executed, the code first needs to be parsed. The parsing process converts the code into a language that the computer can understand and performs syntax checks on the code to ensure its correctness. If the code contains syntax errors, the parser will not execute the code.

2. Create a global execution context

Before executing JavaScript code, you need to create a global execution context. The global execution context is the highest-level execution context in JavaScript, and its scope chain includes all global variables, global functions, and global objects. The global execution context is created when the page is loaded and destroyed when the page is unloaded.

3. Create a function execution context

When you define a function using JavaScript, a function execution context is also created. Similar to the global execution context, the function execution context also contains information such as variables, functions, and objects. However, the scope chain of a function execution context only includes variables and functions within the scope of the function, and does not include the global execution context.

4. Execute JavaScript code

During the execution of JavaScript scripts, the code is executed in a certain order. When JavaScript code is executed, the code is executed based on where it appears in the code. If it is a function call, a new function execution context will be created. If it is ordinary code, the current execution context will be used for code execution.

5. Variable promotion

In JavaScript, variables declared using var are promoted to the top of the current execution context. This means that even if a variable is declared later in the code, the JavaScript engine will still hoist it to the top of the execution context before the code is executed. This process is called variable promotion.

6. Scope chain

In JavaScript, each function has its own scope chain. The scope chain records the location information of all variables and functions in the execution context and links them together in the order of declaration. When a variable is referenced, the JavaScript engine will look along the scope chain until it finds the variable. If the variable does not exist in the scope chain, the variable is considered undefined.

7. Execution Stack

All JavaScript code is executed through the execution stack. The execution stack is a data structure used to store information about all execution contexts. During script execution, whenever a new execution context is created, it is pushed onto the execution stack. When an execution context completes execution, it is popped from the execution stack and execution control is returned to the previous execution context.

In short, the script execution process of JavaScript is very complex and orderly. The reason why JavaScript script execution process can complete various programming tasks is because many complex algorithms and data structures are responsible for supporting it behind the scenes. For a JavaScript developer, it is very important to understand the script execution process of JavaScript, because this is the key to writing high-quality, high-performance JavaScript code.

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