JavaScript 中 let、var 和 const 之间的区别是什么：简单解释-js教程-PHP中文网

JavaScript 中 let、var 和 const 之间的区别是什么：简单解释

王林

发布： 2024-08-21 06:05:32

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What is the Difference Between let, var, and const in JavaScript : Simple Explanation

了解 JavaScript 中 let、var 和 const 之间的区别

在 JavaScript 中，您可以使用 let、var 和 const 声明变量。这些关键字可能看起来相似，但它们具有关键差异，可以显着影响代码的行为方式。在本文中，我们将解释它们之间的差异，并帮助您了解何时使用它们。

var、let 和 const 之间的主要区别

var	let	const
Introduced in: Has been available since the beginning of JavaScript.	Introduced in: Added in ES6 (ECMAScript 2015).	Introduced in: Added in ES6 (ECMAScript 2015).
Scope: Function-scoped. A var variable is accessible throughout the function where it’s declared.	Scope: Block-scoped. A let variable is only accessible within the block {} where it’s declared.	Scope: Block-scoped, just like let.
Hoisting Behavior: var variables are hoisted and can be used before they are declared (though they will be undefined).	Hoisting Behavior: let variables are hoisted but not initialized, so you cannot use them before the declaration.	Hoisting Behavior: Similar to let, const variables are hoisted but not initialized, so they must be declared before use.
Re-declaration: You can re-declare a var variable in the same scope without any errors.	Re-declaration: You cannot re-declare a let variable in the same scope.	Re-declaration: You cannot re-declare a const variable, similar to let.
Reassignment: Variables declared with var can be reassigned.	Reassignment: Variables declared with let can also be reassigned.	Reassignment: Variables declared with const cannot be reassigned; they are constant.

举例说明差异

下面的示例展示了 var、let 和 const 的不同行为：

function userDetails(username) {
  if (username) {
    console.log(salary); // Output: undefined (due to hoisting)
    console.log(age); // Error: ReferenceError: Cannot access 'age' before initialization
    console.log(country); // Error: ReferenceError: Cannot access 'country' before initialization

    let age = 30;
    var salary = 10000;
    const country = "USA";

    // Trying to reassign const
    // country = "Canada"; // Error: Assignment to constant variable.
  }

  console.log(salary); // Output: 10000 (accessible due to function scope)
  console.log(age); // Error: age is not defined (due to block scope)
  console.log(country); // Error: country is not defined (due to block scope)
}
userDetails("John");

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示例说明：

用 var 提升： 用 var 声明的工资变量被提升到函数的顶部。这就是为什么您可以在声明之前访问它，尽管在赋值之前它的值是未定义的。
使用let和const提升：age和country变量也被提升，但与var不同，它们没有初始化。这意味着您无法在声明之前访问它们，从而导致引用错误。
块作用域： 在 if 块之后，由于 var 具有函数作用域，salary 仍然可以访问。然而，age（用let声明）和country（用const声明）都是块作用域的，因此不能在块外访问它们。
用 const 重新赋值： 用 const 声明的变量不能被重新赋值。在示例中，尝试更改国家/地区的值将导致错误。