Boolean arithmetic

[doerwalter]

TypeScript Version: 3.0.3

Search Terms: bool arithmetic

Code

function cmp(a : string, b : string) : number
{
	return (a > b) - (a < b);
}

Expected behavior:

I would have expected that Typescript compiles this source without any complaints, as Javascript treats boolean values true and false as the numbers 1 and 0 in arithmetic expressions, but it seems that Typescript doesn't.

In fact the Javascript version of this function does work as expected:

function cmp(a, b)
{
	return (a > b) - (a < b);
}

cmp("foo", "bar") returns 1 and cmp("bar", "foo") returns -1.

Actual behavior:

Typescript complains with the following error messages:

src/boolarithmetic.ts:3:9 - error TS2362: The left-hand side of an arithmetic operation must be of type 'any', 'number' or an enum type.

3  return (a > b) - (a < b);
          ~~~~~~~

src/boolarithmetic.ts:3:19 - error TS2363: The right-hand side of an arithmetic operation must be of type 'any', 'number' or an enum type.

3  return (a > b) - (a < b);
                    ~~~~~~~

[ghost]

See https://stackoverflow.com/questions/29918324/is-typescript-really-a-superset-of-javascript

The whole purpose of TypeScript is to complain about code that looks fishy, such as relying on implicit conversions. It's easy to change this to an explicit conversion using return Number(a > b) - Number(a < b);.

[RyanCavanaugh]

See also https://stackoverflow.com/questions/41750390/what-does-all-legal-javascript-is-legal-typescript-mean