Describe the bug
Unlike in JavaScript, numeric literals cannot be used as plain object keys.

To Reproduce
math.evaluate('{0: 12-7, 1: 8+2}[2-2]') throws SyntaxError: Symbol or string expected as object key (char 2).

Discussion
It appears that the parser knows exactly what the situation is, which suggests there is no technical reason why this expression couldn't be allowed in the mathjs expression language. Given that it is perfectly legal in JavaScript and evaluates to 5 as expected, I would recommend that the mathjs parser be extended to accept numeric literals as object keys.

In order for this to be useful, the mathjs evaluator will also need to be extended to allow numeric values in indexing into plain objects: currently math.evaluate('{"0": 12-7, "1": 8+2}[2-2]') throws TypeError: Cannot apply a numeric index as object property

Workaround for now
One can convert both the keys and the index expressions to strings: math.evaluate('{"0": 12-7, "1": 8+2}[string(2-2)]') produces 5 as expected. But this is of course cumbersome.