Why if('k' in 42); throw exception while for('k' in 42); does not, in javascript?

Question

From a language design perspective, why does

if ('k' in 42);

throw a TypeError exception while

for ('k' in 42);

does not?

I've read the following sections in ECMAScript spec:

• 11.8.7 The in operator
• 12.6.4 The for-in Statement

Can someone explain the rationale in having such an inconsistency?

Why can't expression 'k' in 42 in if (...) just evaluate to false?

Answer 1

Re-read the in section.

If Type(rval) is not Object, throw a TypeError exception.

42 is not an object, it's a number

typeof 42 // 'number'
typeof new Number(42) // 'object'

for-in statement doesn't require an object, in fact it actually converts it to an object

Let obj be ToObject(experValue).

I don't think it's a reason behind this behaviour, it's probably a design flaw(inconsistency)

Answer 2

I believe the key here is the difference between 'operator' (in) and 'statement' (for-in). If you check the standard, you'll see that the latter only throw Errors when they're written incorrectly. The with case, which throws a SyntaxError (!) in the strict mode, is quite telling.

Apparently you cannot guess whether or not for (var x in someExpr) is written incorrectly unless you evaluate someExpr first.