为什么{} == false抛出异常？ [英] Why does {} == false throw an exception?

问题描述

在IE和Chrome中，在JavaScript控制台中输入一个例外：

In IE and Chrome, typing this into the JavaScript console throws an exception:

{} == false   // "SyntaxError: Unexpected token =="

然而，所有这些语句都被评估没有问题： p>

However, all of these statements are evaluated with no problem:

false == {}   // false

({} == false) // false

var a = {};
a == false    // false

这是有意的行为吗？为什么会发生这种情况？

Is this intentional behavior? Why does this happen?

推荐答案

在控制台中，当您使用 {} ，您不是创建一个对象字面值，而是一个代码块（即与如果语句或循环体）所使用的块相同。然后，显然不需要像 == 这样的符号。

In the console, when you start a statement with {}, you are not creating an object literal, but a code block (i.e. the same block as you would make with an if statement or a loop body). A symbol like == is then obviously not expected afterwards.

如果你想到一个代码块，你知道

If you think of a code block, you know that something like a = 5; could come after it:

if (some_condition) {
    // do something
}
a = 5;

然后，您可以使用它在控制台中进行测试，并发现它工作正常：

You can then use this to test in the console, and find that it works just fine:

{} a = 5;

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