Flowtype constantly requiring null checks

By using a type checker, you are opting into the rules that it enforces. Accessing a property on a nullable type is one of those restrictions. So if you want to have exceptions for null values, you need to explicitly throw to prove to Flow that it is what you want. You could for instance make a module like

if (!document.body) throw new Error("Unexpectedly missing <body>.");
export const body: HTMLElement = document.body;

export function querySelector(el: HTMLElement, selector: string): HTMLElement {
    const result = el.querySelector(selector);
    if (!result) throw new Error(`Failed to match: ${selector}`);
    return result;
}

By throwing, these functions explicitly say "I will return an element" in all cases, and in null cases, they will throw exceptions.

Then in your normal code, you are guaranteed you can use those

import {body, querySelector} from "./utils";

body.appendChild(document.createElement('div'));

querySelector(body, 'div').setAttribute('thing', 'value');

and it will typecheck property.


When I know for sure that my variable won't be null and Flow doesn't, I use an unwrap() function:

export default function unwrap<T>(value: T): $NonMaybeType<T> {
  if (value !== null && value !== undefined) return value
  throw new Error('Unwrapping not possible because the variable is null or undefined!')
}

Tags:

Flowtype

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