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Scope, Constants

var vs let​

tip
  • Variables declared with var have function scope.(Var变量具有函数作用域)
  • the temporal dead zone(零时死区)
如果在函数内使用 var 声明变量，Javascript 引擎会将它们视为在函数作用域的顶部声明。但是，如果该变量已在函数外部声明，则无论实际声明发生在何处，它都具有全局作用域。这就是所谓的吊装。
function logAge() {
    console.log( 'age:', age ); // 这里会输出undefined
    var age = 25;
}
logAge();
name会报错,而var不会
function logName() {
    console.log( 'name:', name ); // ReferenceError: name is not defined
    let name = 'Ben';
}
logName();
danger

作为完整的参考，let、const 和 class 声明存在临时死区。对于 var 、 function 和 function* 声明来说，它不存在。

const​

tip

Declarations with const are block scoped, they have to be initialized, and their value cannot be changed after initialization.(申明必须初始化，之后不可改变)

const PI = 3.1415;
PI = 3.14; // error

let, const, and var​

tip
  • Rule 1: use let for variables, and const for constants whenever possible. Use var only when you have to maintain legacy code.(尽量用let，除非祖传代码)
  • Rule 2: Always declare and initialize all your variables at the beginning of your scope.(每次在作用域顶部定义变量)