Why are some object-literal properties quoted and others not? [duplicate]

Answer 1

for and class are language keywords. Your interpreter would throw a SyntaxError when those are unquoted.

See section 7.6.1.1 in the Spec you linked to.

Answer 2

There is a reason at this point (two plus years later) to quote object literal properties. If one wants to minify their code using the Closure Compiler they may need to make the properties accessible to other source files. In that case, they will want to avoid having symbols renamed by the compiler. By quoting the property name, the Closure Compiler will not minify (rename) them.

See: Removal of code you want to keep

(This applies to at least the ADVANCED_OPTIMIZATIONS setting.)

Answer 3

Javascript has a lot of reserved words that are not actually used by the language which I think were reserved for possible future use. class is one of these even though Javascript does not actually use classes. Another is goto and there's absolutely no chance of that ever being used. The result, however, is that if you want to use these as a json key then it has to be quoted. Strictly speaking you should probably always quote your keys just to avoid the possibility of falling foul of the javascript unused reserved word trap (mind you - I never do).

Answer 4

Javascript language keywords or reserved keywords are always surrounded by quotes in there.

Answer 5

Those are Javascript reserved words, and (though not really necessary) the syntax of the language requires that they be quoted.

Strictly speaking, pure "JSON" notation requires that all of the "key" strings be quoted. Javascript itself however is OK with keys that are valid identifiers (but not reserved words) being unquoted.