How to declare that a var has now been initialized, if Flow can't infer it automatically?

Question

Try this code in Flow's REPL :

const numbers = [5, 3, 22, 2, 6];

let max: number;

numbers.forEach(item => {
  if (!max || item > max) max = item;
});

console.log(max++); // type error

I can be certain that max will have been initialized as a number before the max++ expression. But Flow can't infer this automatically, so it complains.

I want to say to Flow, after the forEach: please assume the max variable has now been initialized. Is there a way to do this?

(Related – Flow has 'declarations': declare var max: number; – this lets you declare that a number variable called var exists in global scope. But you can't use it to redeclare something already declared in your own scope.)

Answer 1

You can initialize max by let max: number = numbers[0]; . That should stop flow from complaining.

Answer 2

You can also rewrite your code in a less mutable way:

const numbers = [5, 3, 22, 2, 6];

const max = numbers.reduce((max, item) => item > max ? item : max,
  Number.NEGATIVE_INFINITY);

console.log(max + 1);

This avoids any kind of mutation, making the life easier for Flow (and probably for humans too :) )