Function wrapReturn

Transform a function from a normal JS function which may return null or undefined to a function which returns a Maybe instead.

For example, dealing with the Document#querySelector DOM API involves a lot of things which can be null:

const foo = document.querySelector('#foo');
let width: number;
if (foo !== null) {
  width = foo.getBoundingClientRect().width;
} else {
  width = 0;
}

const getStyle = (el: HTMLElement, rule: string) => el.style[rule];
const bar = document.querySelector('.bar');
let color: string;
if (bar != null) {
  let possibleColor = getStyle(bar, 'color');
  if (possibleColor !== null) {
    color = possibleColor;
  } else {
    color = 'black';
  }
}
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(Imagine in this example that there were more than two options: the simplifying workarounds you commonly use to make this terser in JS, like the ternary operator or the short-circuiting || or ?? operators, eventually become very confusing with more complicated flows.)

We can work around this with Maybe, always wrapping each layer in Maybe.of invocations, and this is somewhat better:

import Maybe from 'true-myth/maybe';

const aWidth = Maybe.of(document.querySelector('#foo'))
  .map(el => el.getBoundingClientRect().width)
  .unwrapOr(0);

const aColor = Maybe.of(document.querySelector('.bar'))
  .andThen(el => Maybe.of(getStyle(el, 'color'))
  .unwrapOr('black');
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With wrapReturn, though, you can create a transformed version of a function once and then be able to use it freely throughout your codebase, always getting back a Maybe:

import { wrapReturn } from 'true-myth/maybe';

const querySelector = wrapReturn(document.querySelector.bind(document));
const safelyGetStyle = wrapReturn(getStyle);

const aWidth = querySelector('#foo')
  .map(el => el.getBoundingClientRect().width)
  .unwrapOr(0);

const aColor = querySelector('.bar')
  .andThen(el => safelyGetStyle(el, 'color'))
  .unwrapOr('black');
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