`2 == {}`和`{} == 2`之间有什么区别？

Question

When I type 2 == {} on Chrome developer tools, it will return false , but when I type {} == 2 , I get a Error Uncaught SyntaxError: Unexpected token == . What's hell == have done?

Answer 1

{ is syntactically ambiguous. It is used in expressions (object literal syntax, {foo: 42, bar: "hello"} ) and in statements (block, { foo(); bar(); baz(); } ).

If a { occurs at the beginning of a statement, it is always parsed as the start of a block.

Thus

{} == 2

parses as

{
    // empty block
}
== 2  // syntax error

On the other hand, in 2 == {} , the { appears in the middle of an expression and is treated as an object.

This is also how you can make {} == 2 work: Just make sure the { is not the first token, which you can do by eg writing ({}) == 2 or ({} == 2) .

Answer 2

You might wrap it in parentheses to distinguish from a block statement . Then you get an epression .

({} === 2)

Answer 3

if you write {} in javascript, it implies object. You can declare object in braces {}. so {} == 2 or 2=={} is same.and in this case , both statment return false.

Answer 4

In javascript, an object is a value in memory which is referenced by an identifier.
Primitive values can be compared with other types but when you try to compare an undeclared object, it can't be compared as there is no identifier to it. If you first declare it :

var object ={};
object == 2;

This returns false because now there is an identifier to the object being referenced. When you don't declare it, it acts as a block.