js 实现简单的parseInt和parseFloat

function myParseInt(str: string): number {
  let result = NaN;
  for (let i = 0; i < str.length; i++) {
    const dec: number = str.charCodeAt(i);
    if (dec < 48 || dec > 57) break;
    if(isNaN(result)) result = 0;

    const value = dec - 48;
    result = result * 10 + value;
  }
  return result;
}

function myParseFloat(str: string): number {
  let result = NaN;
  let isDecimal = false; // 小数点
  let decimalIndex = 0; // 小数点第几位
  const N = 1000;
  for (let i = 0; i < str.length; i++) {
    const dec: number = str.charCodeAt(i);
    const isPoint = dec === 46;

    if (isPoint && isDecimal) {
      // 再次遇到小数点直接返回
      break;
    } else if (isPoint) {
      isDecimal = true;
      continue;
    }

    if (dec < 48 || dec > 57) break;
    if (isNaN(result)) result = 0;

    const value = dec - 48;

    if (!isDecimal) {
      result = result * 10 + value;
    } else {
      decimalIndex++;
      result = (result * N + (value / 10 ** decimalIndex) * N) / N;
    }
  }
  return result;
}

console.log(myParseInt("12.22")); // 12
console.log(myParseFloat("0.22")); // 0.22