(fix): infer return type in "or" and "and" helpers

[SergeAstapov]

with v4.0.0-beta.0, return type for and and or is boolean which does not match the semantics of the helper itself.

see https://github.com/jmurphyau/ember-truth-helpers/blob/v4.0.0-beta.0/packages/ember-truth-helpers/src/helpers/or.ts#L9

This PR tries to infer the return type from the arguments, kinda similar to what was previously done in https://github.com/Gavant/glint-template-types/blob/v0.3.5/types/ember-truth-helpers/or.d.ts without limiting to 5 arguments.

This still does not work as intended, so more work needed.

[chriskrycho]

You probably want something like this:

import { assert } from '@ember/debug';

/**
 * Return first truthy value, otherwise the last value
 *
 * @param positional - array of values
 */
export default function or<T extends Array<unknown>>(
  ...positional: T
): T[number] {
  assert(
    `must pass 2 or more arguments to 'or' helper, passed ${positional.length}`,
    positional.length >= 2
  );

  // Falling back to the final value makes this helper behave the same way as
  // the JavaScript `||` operator.
  return positional.find(Boolean) || positional[positional.length - 1];
}

function and<T extends unknown[]>(...params: T): T[number] {
  assert('`and` helper expects multiple parameters', params.length > 1);
  return params.reduce((a, b) => a && b, true);
}

You probably want something like this:

import { assert } from '@ember/debug';

/**
 * Return first truthy value, otherwise the last value
 *
 * @param positional - array of values
 */
function or<T extends Array<unknown>>(
  ...positional: T
): T[number] {
  assert(
    `must pass 2 or more arguments to 'or' helper, passed ${positional.length}`,
    positional.length >= 2
  );

  // Falling back to the final value makes this helper behave the same way as
  // the JavaScript `||` operator.
  return positional.find(Boolean) || positional[positional.length - 1];
}

function and<T extends unknown[]>(...params: T): T[number] {
  assert('`and` helper expects multiple parameters', params.length > 1);
  return params.reduce((a, b) => a && b, true);
}

Given you're shipping a breaking 4.0 release, and that there is a polyfill with support for functions-as-helpers which supports 3.28 LTS, I think you can and should just use these. If, however, you decide there is a reason to stick with helper, you will need to change it slightly, but do not have to ship a standalone Signature (we went out of our way to make sure inference works correctly with an "inline" definition):

import { helper } from '@ember/component/helper';

// or.ts
const or = helper(function or<T extends Array<unknown>>(
  positional: T
): T[number] {
  // same implementation ...
});

// and.ts
export default helper(function and<T extends unknown[]>(
  positional: T
): T[number] {
  // same implementation ...
});

[SergeAstapov]

we went slightly different route with #188