直接访问空对象{}上的属性时的语法错误 [英] Syntax Error when directly access attribute on empty object {}

问题描述

> "[object Number]" === Object.prototype.toString.call(1) // #1
< true
> "[object Number]" === {}.toString.call(1)               // #2
< true
> {}.toString.call(1) === "[object Number]"               // #3
< SyntaxError: Unexpected token '.'
> ({}).toString.call(1) === "[object Number]"             // #4
< true
> {}.toString.call(1)                                     // #5
< SyntaxError: Unexpected token '.'
> !{}.toString.call(1)                                    // #6
< false
> test = {}.toString.call(1)                              // #7
< "[object Number]"

从样本中您可以看到，#2和#3几乎相同，只是它们的左侧和右侧互换了.#2工作正常，但#3给出语法错误.为了使#3正常工作，需要使用一对括号.另外，从#5-7可以看到，只要 {} 不在最左边，它就可以正常工作.

As you can see from the sample, #2 and #3 are almost identical except that the they have the left and right side exchanged. #2 works fine but #3 gives a syntax error. To get #3 to work, a pair of parentheses is required. Additionally, from #5-7 we can see that as long as {} is not at the left most, it works fine.

但是为什么呢?

推荐答案

在#1中，当找到 rvalue 和运算符 === 时，javascript认为 lvalue 可以是值或表达式.

In #1, when it finds rvalue and an operator ===, javascript considers that the lvalue could be a value or an expression.

在#2中，与#1相同.因此 {} 被视为对象文字

In #2, same as #1. hence {} is considered as an object literal

在#3中，由于javascript从右到左评估该语句，因此 {} 只是花括号，而不是对象文字.因此#3不起作用，因为您不会在花括号上使用 toString 函数.

In #3, since javascript evaluates the statement from right to left, the {} is considered just as a curly brace but not an object literal. hence #3 is not working because you won't be getting toString function on curly braces.

在#4中，当将 {} 括在()分组运算符内时，javascript认为它是一个表达式.因此#4之所以有效，是因为 {} 被评估为对象文字，并且 toString 可用.

In #4, when you enclose {} within () grouping operator, javascript thinks it is an expression. hence #4 works because {} is evaluated as an object literal and toString be available.

在#5中，与#3相同.javascript认为 {} 只不过是括号，因为没有表达式或表达式.

In #5, same as #3. javascript thinks that {} as just a brace since there is not expressions or with it.

在#6中，与#4相同.有一个表达式！.因此被评估为对象

In #6, same as #4. there is an expression ! . hence is evaluated as an object

在#7中，与#4相同.有一个赋值运算符，因此将其作为表达式求值.

In #7, same as #4. there is an assignment operator, hence is evaluated as an expression.

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