array-prototype-findvalue.mjs

import * as assert from 'assert/strict';

/**
 * I’m not sure if this is practically useful but:
 * The single utility method .findValue() for Arrays can be used
 * to implement several existing Array methods.
 */
Array.prototype.findValue = function (cb, defaultResult, thisValue) {
  thisValue ??= this;
  for (let i=0; i<this.length; i++) {
    const result = cb .call(thisValue, this[i], i, this);
    if (result !== null && typeof result === 'object' && 'value' in result) {
      return result.value;
    }
  }
  return defaultResult;
};

////////// .find((x) => x > 0) //////////

assert.equal(
  [-1, 0, -7, 123].findValue((x) => x > 0 ? {value: x} : undefined, undefined),
  123
);
assert.equal(
  [-1, 0, -7].findValue((x) => x > 0 ? {value: x} : undefined, undefined),
  undefined
);

////////// .findIndex((x) => x > 0) //////////

assert.equal(
  [-1, 0, -7, 123].findValue((x, index) => x > 0 ? {value: index} : undefined, -1),
  3
);
assert.equal(
  [-1, 0, -7].findValue((x, index) => x > 0 ? {value: index} : undefined, -1),
  -1
);  

////////// Return both value and index at the same time //////////

assert.deepEqual(
  [-1, 0, -7, 123].findValue((x, index) => x > 0 ? {value: {index, value:x}} : undefined, undefined),
  { index: 3, value: 123 }
);
assert.equal(
  [-1, 0, -7].findValue((x, index) => x > 0 ? {value: {index, value:x}} : undefined, undefined),
  undefined
);

////////// .some((x) => x.length > 0) //////////

assert.equal(
  ['', 'a', ''].findValue((x) => x.length > 0 ? {value:true} : undefined, false),
  true
);
assert.equal(
  ['', '', ''].findValue((x) => x.length > 0 ? {value:true} : undefined, false),
  false
);

////////// .every((x) => x.length > 0) //////////

assert.equal(
  ['', 'a', ''].findValue((x) => x.length === 0 ? {value:false} : undefined, true),
  false
);
assert.equal(
  ['a', 'b', 'c'].findValue((x) => x.length === 0 ? {value:false} : undefined, true),
  true
);