JavaScript array and object addition

Question

I'm testin some JavaScript concepts and I'm trying to understand why:

typeof({} + {})
typeof([] + [])

return a 'string' yet:

typeof {}
typeof []

return an 'object'

I've tried this in NodeJS, FireFox, Chrome and IE9 all with the same results.

Your observations all have to do with the semantics of the + operator, as is easily confirmed by reading the language spec. — Pointy
– Pointy, Commented Nov 15, 2012 at 14:52
I'm guessing that because the addition of objects and arrays make no sense to the compiler/interpreter that it's doing a toString() on these objects before concatenating them. Is this right? — Guy
– Guy, Commented Nov 15, 2012 at 14:53
Interestingly, on the Chrome console, {} + {} gives NaN and typeof(NaN) give "number" — Dancrumb
– Dancrumb, Commented Nov 15, 2012 at 14:54
@Dancrumb {} + {} is not the same as ({} + {}). The first {} is treated as an empty block, so it is equivalent to +{}. With unary +, the operand is always converted to a number. — Esailija
– Esailija, Commented Nov 15, 2012 at 14:56

Esailija · Accepted Answer · 2012-11-15 14:54:28Z

4

The + operator is defined to either concatenate strings or add numbers. If you try something else, the operands have to be converted to strings or numbers first. It looks like string is preferred here.

You can control what string or number is returned by defining functions .toString and .valueOf on the objects respectively

answered Nov 15, 2012 at 14:54

Esailija

140k24 gold badges280 silver badges328 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Felix · Accepted Answer · 2012-11-15 14:53:36Z

1

You can't concatenate or "add" arrays or objects using the + operator.

If you add two objects their string-representations are concatenated.

answered Nov 15, 2012 at 14:53

Felix

5334 silver badges11 bronze badges

Comments

jbabey · Accepted Answer · 2012-11-15 14:58:15Z

0

The + operator attempts to coerce the operands to a common type. In this case, the type is string, since adding objects or arrays does not make any sense.

From the + operator documentation:

If Type(lprim) is String or Type(rprim) is String, then Return the String that is the result of concatenating ToString(lprim) followed by ToString(rprim)

Since the default value of an object is a string (an empty string, in the case of an empty object), the return value of the operation is '' + '' => ''.

See the ECMAScript documentation for the + operator, ToPrimitive, and an object's DefaultValue if you care about the internals.

answered Nov 15, 2012 at 14:58

jbabey

46.7k12 gold badges73 silver badges94 bronze badges

Collectives™ on Stack Overflow

JavaScript array and object addition

3 Answers 3

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related