myParseInt.js

function myParseInt(input) {
  let result = 0;
  let isNegative = false;
  let i = 0;

  input = input.trim();

  if (input[0] === '-') {
    isNegative = true;
    i = 1;
  } else if (input[0] === '+') {
    i = 1;
  }

  if (/\D/.test(input[i])) {
    return NaN;
  }

  const ZERO_ASCII_CODE = '0'.charCodeAt(0);
  for (; i < input.length; i++) {
    const char = input[i];
    if (/\D/.test(char)) {
      break;
    }

    result = result * 10 + (char.charCodeAt(0) - ZERO_ASCII_CODE);
  }

  if (isNegative) {
    result = -result;
  }

  return result;
}

const testInputs = [
  '123',
  '  456  ',
  '-789',
  '+1011',
  '  -2222  ',
  '42abc',
  'abc42',
  '',
  '   ',
  '+',
  '-',
  '0',
  '-0',
];

testInputs.forEach((input, index) => {
  const myResult = myParseInt(input);
  const nativeResult = parseInt(input);

  console.log(`----- Test ${index + 1} -----`);
  console.log(`                 Input: "${input}"`);
  console.log('Native parseInt result:', nativeResult);
  console.log('    My parseInt result:', myResult);
});