day1：js字符串方法slice的实现逻辑

js字符串方法slice的实现逻辑:

const str = '123213';
  String.prototype._slice = function (start = 0, end = this.length) {
    if (!start && end === this.length) {
      return this;
    }
    start *= 1;
    end *= 1;
    if (isNaN(start)) {
      throw new Error('start is not a number')
    }
    if (isNaN(end)) {
      throw new Error('end is not a number')
    }
    let str = '';
    if (start > end) {
      return str;
    }
    let startIndex = start,
      endIndex = end;

    if (startIndex < 0) {
      startIndex = start + this.length
    }

    if (endIndex < 0) {
      endIndex = end + this.length
    }

    for (let i = startIndex; i < endIndex; i++) {
      str += this[i];
    }
    return str;
  }
  console.log(str.slice(0, 3));
  // 输出结果：123