Adding unary operator + to transform string into numbers

[nicola]

When reading the test files, I spot that parser.spec.js contained:

it('should parse unary - expressions', () => {
  expect(evaluate("-1")).toEqual(-1);
  expect(evaluate("+1")).toEqual(1);

So I divided them in two tests:

1. should parse unary - expressions
2. should parse unary + expressions

I realised that the unary + can be used to transform expressions of any type into numbers returning a number or NaN, see Mozilla Dev Network.

Therefore I implemented PrefixPlus similarly to PrefixNot. Here are the new tests that previously did not pass:

    it('should parse unary + expressions', () => {
      expect(evaluate("+1")).toEqual(1);
      expect(evaluate("+'1'")).toEqual(1);
      expect(evaluate("+'not a number'")).toEqual(NaN);
    });

However I am not sure if the angular team wants to add this level of complexity.

[MikeMcElroy]

I'm uncomfortable with this -- I don't think it's a good idea for Expressionist to re-implement Javascript's funky type coercion you have in the second expectation, but I think there should be a test case to document what happens if someone tries to + or - a NaN-value. To that end, what should be the behavior around that circumstance?

[nicola]

I would personally expect to always have a javascript behavior in there.

[caitp]

expressions are more about data binding, the primary requirements of the expression parser are really just dereferencing properties of objects and calling functions (implemented in a controller somewhere). Negation, okay, but things like >>> 0 or +foo etc are a bit silly and out of scope for this, imo.