Scope chain in JavaScript

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The Scope Chain in JavaScript is a mechanism for looking up variables created by functions and their nested functions. The scope chain determines where variables can be accessed, and the order in which variables are accessed.

The scope chain exists based on the nested structure of functions. Whenever a variable is declared in a function, the JavaScript engine adds the variable to the scope chain of the current scope. When code needs to find the value of a variable, the JavaScript engine first looks in the scope of the current function, and if not found, searches upward through nested external functions until the variable is found or the global scope is reached.

Here are some key concepts and examples of scope chains:

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⭐ Local Scope

Variables declared inside a function have local scope and can only be accessed inside the function.

function outer() {
    
    
  var x = 10; // 局部作用域
  function inner() {
    
    
    console.log(x); // 可以访问外部函数的变量
  }
  inner();
}
outer();

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⭐ Nested Scope

Functions can be nested, and inner functions can access variables of outer functions, but not vice versa.

function outer() {
    
    
  var x = 10; // 外部函数的变量
  function inner() {
    
    
    var y = 20; // 内部函数的变量
    console.log(x); // 可以访问外部函数的变量
  }
  inner();
  console.log(y); // 无法访问内部函数的变量，会引发错误
}
outer();

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⭐ Search order of scope chain

When looking for variables, the JavaScript engine looks in order from the inside out of the scope chain. If a matching variable is found, the search stops.

var x = 10; // 全局变量
function outer() {
    
    
  var x = 20; // 外部函数的变量
  function inner() {
    
    
    console.log(x); // 会首先查找内部函数的变量，输出 20
  }
  inner();
}
outer();

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⭐ Global Scope

If the variable is not found in any of the nested functions, the JavaScript engine looks in the global scope.

var x = 10; // 全局变量
function outer() {
    
    
  function inner() {
    
    
    console.log(x); // 会查找全局变量，输出 10
  }
  inner();
}
outer();

Understanding the scope chain is very important for writing complex JavaScript programs. It helps determine the accessibility and visibility of variables, avoid naming conflicts, and optimize the performance of the code.

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⭐ Written at the end

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