FlowType: null check fails

Question

Why does the null-check fail in func1 while it is ok in func2

/* @flow */

const func1 = (arr?: Array<*>) => {
  const isArrayNotEmpty = arr && arr.length;

  if (isArrayNotEmpty) {
    arr.forEach((element) => console.log(element));
  }
}

const func2 = (arr?: Array<*>) => {
  if (arr && arr.length) {
    arr.forEach((element) => console.log(element));
  }
}

Live example

Answer 1

I don't know the reasons why Flow doesn't support this, but it doesn't. It currently requires that checks for type refinements actually happen in the if statement, and does not track them if they are abstracted out into a separate variable.

There is an alternative that may be acceptable to you (I'm not sure it's documented anywhere):

/* @flow */

const func1 = (arr?: Array<*>) => {
  if (isArrayNotEmpty(arr)) {
    arr.forEach((element) => console.log(element));
  }
}

function isArrayNotEmpty(x: mixed): %checks {
  return x && x.length;
}

(tryflow)

The special %checks return type indicates to Flow that it should look into the body of the function to figure out what it implies about the types of the variables it is passed. I believe there are some restrictions on what can be in the body of such a function. It may even be the case that it just has to return a single expression. This should give you enough to experiment with it, though.