在 JavaScript 中，链式赋值好吗? [英] In JavaScript, is chained assignment okay?

问题描述

我对 JS 或其语法并不陌生，但有时，该语言的语义有时让我感到困惑.今天上班，同事提到了这个:

Am not new to JS or its syntax, but sometimes, the semantics of the language has me stumped at times. At work today, a colleague mentioned this:

var a = b = [];

与

var a = [], b = [];

或

var a = []; var b = [];

因为第一个版本实际上将空数组的引用分配给 a 和 b.我不能完全接受这是真的，但我不确定.大家怎么看?

since the first version actually assigns the reference to an empty array to a and b. I couldn't quite accept this as true, but I'm not sure. What do you all think?

推荐答案

是的，它们不一样.var a = b = [] 等价于

Yes, they're not the same. var a = b = [] is equivalent to

var a;
b = [];
a = b;

不仅 a 和 b 都被分配了相同的值(对同一个空数组的引用)，b 没有被声明根本.在 ECMAScript 5 及更高版本的严格模式中，这将抛出一个 ReferenceError;否则，除非在作用域中已经有一个变量 b，b 被静默创建为全局对象的一个​​属性，并且无论代码在哪里，它的行为都类似于全局变量，即使在函数内部.哪个不好.

Not only do both a and b get assigned the same value (a reference to the same empty array), b is not declared at all. In strict mode in ECMAScript 5 and later, this will throw a ReferenceError; otherwise, unless there is already a variable b in scope, b is silently created as a property of the global object and acts similarly to a global variable, wherever the code is, even inside a function. Which is not good.

您可以很容易地看到这一点:

You can see this quite easily:

(function() {
    var a = b = [];
})();

console.log(b); // Shows []

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