Can't filter types with Array.filter

[funtaps]

It's impossible to filter element by type with Array.filter.
Here is example of valid code, that flow don't want to accept.

// @flow
const input: Array<number|string> = [1, 'a', 2, 3, 'b'];
const result: Array<number> = input.filter(el => {
  return typeof el == 'number';
});

It returns

3: const result: Array<number> = input.filter(el => {
                                  ^ call of method `filter`
2: const input: Array<number|string> = [1, 'a', 2, 3, 'b'];
                                  ^^^^^^ string. This type is incompatible with
3: const result: Array<number> = input.filter(el => {
                        ^^^^^^ number

Flow version 0.26.0

[avikchaudhuri]

Duplicate of #1414

[gcanti]

@funtaps a possible workaround:

const input: Array<number|string> = [1, 'a', 2, 3, 'b']
const result = ((input.filter(el => {
  return typeof el == 'number'
}): Array<any>): Array<number>)

[bsmith-cycorp]

Really wish they would solve this. In the meantime, here's a less-gross-but-still-hacky version of the above that we use when we know something is a type but we can't communicate that to Flow:

function cast<T>(val: any): T {
  return val;
}

const input: Array<number|string> = [1, 'a', 2, 3, 'b']
const result = cast<Array<number>>(input.filter(el => typeof el === 'number'))