Why does {} == false throw an exception?

问题:

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

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

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

false == {}   // false

({} == false) // false

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

Is this intentional behavior? Why does this happen?

回答1:

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.

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;

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

{} a = 5;