javascript compilation process

Javascript has become an essential skill for web development. It is not only limited to the browser, but also widely used in various application scenarios such as Node.js, mobile terminals, desktop terminals and embedded devices. During the execution of Javascript, there is a very important concept, which is the compilation process. This article will introduce the execution process of Javascript from the perspective of the compilation process.

What is the compilation process?

The compilation process is the process of converting source code into executable code. In Javascript, the compilation process can be divided into two phases: the parsing phase and the execution phase.

Parsing stage

The parsing stage is divided into two parts: lexical analysis and syntax analysis.

Lexical analysis

The task of lexical analysis is to decompose the source code into lexical units (tokens), which are the smallest units in the language. Lexical units include keywords (such as function, var), identifiers (such as variable names and function names), and operators (such as , -, *, /, etc.).

For example, the following is a simple function declaration:

function add(x, y) {
  return x + y;
}

This function declaration will be broken down into the following lexical units:

Keyword 'function'
Identifier 'add'
Punctuator '('
Identifier 'x'
Punctuator ','
Identifier 'y'
Punctuator ')'
Punctuator '{'
Keyword 'return'
Identifier 'x'
Punctuator '+'
Identifier 'y'
Punctuator ';'
Punctuator '}'

Grammar analysis

The task of syntax analysis is to convert lexical units into abstract syntax trees (AST). Abstract syntax tree is a data structure that represents the syntax structure of code in a tree structure. Each node represents a construct in the grammatical structure, and the child nodes represent the sub-constructs of the construct.

For example, for the above function declaration, the generated abstract syntax tree is as follows:

{
  "type": "FunctionDeclaration",
  "id": {
    "type": "Identifier",
    "name": "add"
  },
  "params": [
    {
      "type": "Identifier",
      "name": "x"
    },
    {
      "type": "Identifier",
      "name": "y"
    }
  ],
  "body": {
    "type": "BlockStatement",
    "body": [
      {
        "type": "ReturnStatement",
        "argument": {
          "type": "BinaryExpression",
          "operator": "+",
          "left": {
            "type": "Identifier",
            "name": "x"
          },
          "right": {
            "type": "Identifier",
            "name": "y"
          }
        }
      }
    ]
  }
}

Execution phase

After the parsing phase is completed, the Javascript program enters the execution phase. The task of the execution phase is to execute the abstract syntax tree.

The process of executing the abstract syntax tree can be divided into two stages: pre-compilation and execution.

Precompilation

The task of precompilation is to process variable and function declarations. Before the execution phase begins, the Javascript engine scans the abstract syntax tree of the program and creates corresponding variables and functions in the current scope. This process is called scope preprocessing.

First, all function declarations are advanced. This process is called Function Declarations Hoisting. The essence of function declaration hoisting is to raise the function name and function body to the top of the current scope, so the function can be called before the function is declared.

For example, the following code:

add(1, 2);

function add(x, y) {
  return x + y;
}

is equivalent to:

function add(x, y) {
  return x + y;
}

add(1, 2);

Secondly, declare all variables in advance. This process is called variable hoisting. It should be noted that variable promotion only promotes the declaration of the variable, but not the assignment statement of the variable.

For example, the following code:

console.log(a);
var a = 1;

is equivalent to:

var a;
console.log(a);
a = 1;

Execution

The execution process is executed in the order in which the statements appear. During execution, variables are assigned, functions are called, and may be scoped by code blocks or functions.

Summary

The compilation process of Javascript includes the parsing phase and the execution phase. The parsing phase includes lexical analysis and syntax analysis, and the execution phase includes precompilation and execution. Before the execution phase, the Javascript engine will preprocess the scope, perform function declaration hoisting and variable hoisting, and then execute the code in the order in which the statements appear.

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