{}

Questions

Why?

• +
• valueOf

Primitive value

typecast operands before evaluation

// typecast obj to boolean
!!obj
            

Array's primitive value

var arr = [];
// still not a primitive value
arr.valueOf() === arr;
            

[] + {}
// trying to get primitive value by valueOf
= ([]).valueOf() + ({}).valueOf()
// previous attempt fails, trying typecast instead
= ([]).toString() + ({}).toString()
= "" + "[object Object]"
= "[object Object]"
            

Ponder this

[] + {valueOf: function valueOf() { return 2}}
            

So far so good

[] + {valueOf: function valueOf() { return 2}}
= "" + {valueOf: function valueOf() { return 2}}.valueOf()
= "" + 2
= "2"
            

Ponder this

{valueOf: function valueOf() { return 2}} + {valueOf: function valueOf() { return 2}}
            

What?

{valueOf: function valueOf() { return 2}} + {valueOf: function valueOf() { return 2}}

= 2
            

code block
&
Labelled Statement

Code block

{ StatementList } Return the result of evaluating StatementList.

var a = 1, b = 2;
{a = 2; b = 3};
console.log(a, b);
            

Code block is everywhere

// this is OK
[].toString();
// this will throw an error
{}.hasOwnProperty();

// this is OK. Wait, what?
{}hasOwnProperty();
            

Code block is everywhere

The leading {} is treated as a code block in browsers.

{}.hasOwnProperty();
// equals to
.hasOwnProperty();
            

{}hasOwnProperty();
// equals to
hasOwnProperty();

// Chrome has an window.hasOwnProperty :)
            

Be careful with
code block

// There are many ways to fool the browser
// so that {} won't be treated as a block
// wrap it in ()
({}).hasOwnProperty();
// add something in front of {}
"",{}.hasProperty();
            

Compression tools "fix" this for us.

Labelled Statement

A statement may be prefixed by a label.

Labelled statements are only used in conjunction with labelled break and continue statements.

loop: for (var i = 0; i < 1000; i++) {
    if (i === 233) break loop;
}
// i = 233
console.log(i);
            

Labelled Statement

Almost any statement can be prefixed by a label.

pointless: var a = 1;
            

Label doesn't do anything in this case.

So we have...

{} + []
// processing code block
= +[]
// trying to get primitive value
= + [].valueOf()
= + [].toString()
= + ""
// typecast
= Number("")
= 0
            

Revealing the trick

{valueOf: function valueOf() { return 2}} + {valueOf: function valueOf() { return 2}}
            

The first part is treated as a code block with a label and a statement inside.

// This first part
{valueOf: function valueOf() { return 2}}
// { label: statement }
// label: "valueOf"
// statement: function valueOf() { return 2}
            

It doesn't return anything.

Revealing the trick

{valueOf: function valueOf() { return 2}} + {valueOf: function valueOf() { return 2}}
            

The second part is an object with a customized valueOf method.

The answer is +2 = 2

Similarly...

{} + [] = +[] = 0 // in browser, not Node.js
{} + {} = +{} = NaN
{} + new Date = 1393349722245
[] + new Date = "Wed Feb 26 2014 01:35:28 GMT+0800 (KRAT)"
[] + + new Date = 1393349722245
            

Further Reading

• ECMA Spec
• What is {} + {} in JavaScript?

Thanks!