为什么在赋值中表达为左侧并不总是有效? [英] Why expression as left-hand side in assignment doesn't always work?

问题描述

此代码有效:

let foo;

(foo) = 'bar';

console.log(foo);

这让我相信任何表达式都可以作为赋值的左侧，但是，这段代码不起作用:

And that makes me believe that any expression works as a left-hand side in assignment, however, this code doesn't work:

let foo1 = "bar";
let foo2;
let foo3;
(foo1 && foo2 && foo3) = "foobar"; // The left-hand side evaluates to undefined which can't be assigned

console.log(foo1, foo2, foo3);

什么使左侧在赋值中有效，为什么它不能总是表达式?

What makes a left-hand side valid in an assignment, why it can't always be an expression?

推荐答案

规范的相关部分是 此处:

LeftHandSideExpression 的AssignmentTargetType 不简单是语法错误.

It is a Syntax Error if AssignmentTargetType of LeftHandSideExpression is not simple.

和此处.

简化版本是(如果我们不考虑解构)您可以为变量或对象的属性赋值.

The simplified version is that (if we leave destructuring aside) you can assign a value to a variable or to a property of an object.

给定(foo)，括号没有意义，表达式只是一个变量名.这很好.

Given (foo), the parenthesis are pointless and the expression is just a variable name. This is fine.

给定 (foo1 && foo2 && foo3) 从变量中读取值，使用 && 运算符和结果进行比较是一个值.

Given (foo1 && foo2 && foo3) the values are read from the variables, compared using the && operator and the outcome is a value.

不能给值赋值，只能给属性或变量赋值.

You can't assign a value to a value, only to a property or a variable.

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