JavaScript中的变量范围是什么？ [英] What is the scope of variables in JavaScript?

问题描述

javascript中的变量范围是什么？它们的内部是否与函数外部相同？或者甚至重要吗？另外，如果变量是全局定义的，那么它们存储在哪里？

What is the scope of variables in javascript? Do they have the same scope inside as opposed to outside a function? Or does it even matter? Also, where are the variables stored if they are defined globally?

推荐答案

我认为我能做的最好的就是给你一个一堆例子来研究。
Javascript程序员实际上根据他们理解范围的程度来排名。
它有时可能非常违反直觉。

I think about the best I can do is give you a bunch of examples to study. Javascript programmers are practically ranked by how well they understand scope. It can at times be quite counter-intuitive.

1. 全局范围的变量

// global scope
var a = 1;

function one() {
  alert(a); // alerts '1'
}

本地范围

// global scope
var a = 1;

function two(a) { // passing (a) makes it local scope
  alert(a); // alerts the given argument, not the global value of '1'
}

// local scope again
function three() {
  var a = 3;
  alert(a); // alerts '3'
}

中级：没有JavaScript中的块范围（ES5; ES6引入 让 ）

Intermediate: No such thing as block scope in JavaScript (ES5; ES6 introduces let)

a。

var a = 1;

function four() {
  if (true) {
    var a = 4;
  }

  alert(a); // alerts '4', not the global value of '1'
}

b 。

var a = 1;

function one() {
  if (true) {
    let a = 4;
  }

  alert(a); // alerts '1' because the 'let' keyword uses block scoping
}

中级：对象属性

var a = 1;

function Five() {
  this.a = 5;
}

alert(new Five().a); // alerts '5'

高级：关闭

var a = 1;

var six = (function() {
  var a = 6;

  return function() {
    // JavaScript "closure" means I have access to 'a' in here,
    // because it is defined in the function in which I was defined.
    alert(a); // alerts '6'
  };
})();

高级：基于原型的范围解析

var a = 1;

function seven() {
  this.a = 7;
}

// [object].prototype.property loses to
// [object].property in the lookup chain. For example...

// Won't get reached, because 'a' is set in the constructor above.
seven.prototype.a = -1;

// Will get reached, even though 'b' is NOT set in the constructor.
seven.prototype.b = 8;

alert(new seven().a); // alerts '7'
alert(new seven().b); // alerts '8'

---

全球+本地：一个额外的复杂案例

var x = 5;

(function () {
    console.log(x);
    var x = 10;
    console.log(x); 
})();

这将打印出 undefined 和 10 而不是 5 和 10 因为JavaScript总是移动变量声明（未初始化）到范围的顶部，使代码等效于：

This will print out undefined and 10 rather than 5 and 10 since JavaScript always moves variable declarations (not initializations) to the top of the scope, making the code equivalent to:

var x = 5;

(function () {
    var x;
    console.log(x);
    x = 10;
    console.log(x); 
})();

Catch子句范围变量

var e = 5;
console.log(e);
try {
    throw 6;
} catch (e) {
    console.log(e);
}
console.log(e);

这将打印出 5 ， 6 ， 5 。在catch子句中 e 隐藏全局变量和局部变量。但是这个特殊范围仅适用于捕获的变量。如果在catch子句中写入 var f; ，那么它与在try-catch块之前或之后定义它的方式完全相同。

This will print out 5, 6, 5. Inside the catch clause e shadows global and local variables. But this special scope is only for the caught variable. If you write var f; inside the catch clause, then it's exactly the same as if you had defined it before or after the try-catch block.

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