function dragstart_handler(ev) {
  // 添加拖拽数据
  ev.dataTransfer.setData("text/plain", ev.target.innerText);
  ev.dataTransfer.setData("text/html", ev.target.outerHTML);
  ev.dataTransfer.setData(
    "text/uri-list",
    ev.target.ownerDocument.location.href
  );
}

function dragstart_handler(ev) {
  // Create an image and then use it for the drag image.
  // NOTE: change "example.gif" to a real image URL or the image
  // will not be created and the default drag image will be used.
  var img = new Image();
  img.src = "example.gif";
  ev.dataTransfer.setDragImage(img, 10, 10);
}

function dragstart_handler(ev) {
  ev.dataTransfer.dropEffect = "copy";
}

var tmp = new Date();

function f() {
console.log(tmp);
if (false) {
var tmp = ‘hello world’;
}
}
f();

let a = "hey I am outside";
if (true) {
  console.log(a); //Uncaught ReferenceError: a is not defined
  let a = "hey I am inside";
}

export default function json_parse() {
  //这是一个能把JSON 字符串转化为javascript数据结构的函数
  //他是一个简单的递归降序的解析器
  var at, //当前字符索引
    ch, //当前字符
    escapee = {
      '"': '"',
      "\\": "\\",
      "/": "/",
      b: "b",
      f: "\f",
      n: "\n",
      r: "\r",
      t: "\t",
    },
    text,
    //当某处出错时，调用error
    error = (m) => {
      throw {
        name: "SyntaxError",
        message: m,
        at: at,
        text: text,
      };
    },
    next = (c) => {
      //如果提供了参数c,那么检测它是否匹配当前字符
      if (c && c !== ch) {
        error("Expected'" + c + "' instead of '" + ch + "'");
      }
      //获取下一个字符，当没有下一个字符时返回空字符串
      ch = text.charAt(at);
      at += 1;
      return ch;
    },
    number = () => {
      //解析一个数字值
      var number,
        string = "";
      if (ch === "-") {
        string = "-";
        next("-");
      }
      while (ch >= "0" && ch <= "9") {
        string += ch;
        next();
      }
      if (ch === ".") {
        string += ".";
        while (next() && ch >= "0" && ch <= "9") {
          string += ch;
        }
      }
      if (ch === "e" || ch === "E") {
        string += ch;
        next();
        if (ch === "-" || ch === "+") {
          string += ch;
          next();
        }
        while (ch >= "0" && ch <= "9") {
          string += ch;
          next();
        }
      }
      number = +string;
      if (isNaN(number)) {
        error("Bad Number");
      } else {
        return number;
      }
    },
    string = () => {
      var hex, i, string, uffff;
      //当解析字符串值时，我们必须找到“和\.
      if (ch === '"') {
        while (next()) {
          if (ch === '"') {
            next();
            return string;
          } else if (ch === "\\") {
            next();
            if (ch === "u") {
              uffff = 0;
              for (let i = 0; i < 4; i++) {
                hex = parseInt(next(), 16);
                if (!isFinate(hex)) {
                  break;
                }
                uffff = uffff * 16 + hex;
              }
              string += String.fromCharCode(uffff);
            } else if (typeof escapee[ch] === "string") {
              string += escapee[ch];
            } else {
              break;
            }
          } else {
            string += ch;
          }
        }
      }
      error("Bad Strng!");
    },
    //跳过空白
    white = () => {
      while (ch && ch <= " ") {
        next();
      }
    },
    word = () => {
      //true ,false,or null
      switch (ch) {
        case "t":
          next("t");
          next("r");
          next("u");
          next("e");
          return true;
        case "f":
          next("f");
          next("a");
          next("l");
          next("s");
          next("e");
          return false;
        case "n":
          next("n");
          next("u");
          next("l");
          next("l");
          return null;
      }
      error("Unexpected '" + ch + "'");
    },
    value, //值函数的占位符
    //解析一个数组值
    array = () => {
      if (ch === "[") {
        next("[");
        white();
      }
      if (ch === "]") {
        next("]");
        return array; //空数组
      }
      while (ch) {
        array.push(value());
        white();
        if (ch === "]") {
          next("]");
          return array;
        }
        next(",");
        white();
      }
      error("Bad array");
    },
    object = () => {
      var key,
        object = {};
      if (ch === "{") {
        next("{");
        white();
        if (ch === "}") {
          next("}");
          return object; //空对象
        }
        while (ch) {
          key = string();
          white();
          next(":");
          object[key] = value();
          white();
          if (ch === "}") {
            next("}");
            return object;
          }
          next(",");
          white();
        }
      }
      error("Bad Object");
    },
    value = () => {
      //解析一个JSON值，他可以是对象，数组，字符串，数字或一个词。
      white();
      switch (ch) {
        case "{":
          return object();
        case "[":
          return array();
        case '"':
          return string();
        case "-":
          return number();
        default:
          return ch >= "0" && ch <= "9" ? number() : word();
      }
    };
  return (source, reviver) => {
    var result;
    text = source;
    at = 0;
    ch = " ";
    result = value();
    white();
    if (ch) {
      error("Syntax error");
    }
    //如果存在reviver函数，我们就递归的对这个新结构调用walk函数，
    //开始时先创建一个临时的启动对象，并以一个空字符串作为键名保存结果，
    //然后传递每个”键/值“ 对给reviver函数去处理可能存在的转换，
    //如果没有reviver函数，我们就简单的返回这个结果
    return typeof reviver === "function"
      ? (function walk(holder, key) {
          var k,
            v,
            value = holder[key];
          if (value && typeof value === "object") {
            for (key in value) {
              if (Object.hasOwnProperty.call(value, k)) {
                v = walk(value, k);
                if (v !== undefined) {
                  value[k] = v;
                } else {
                  delete value[k];
                }
              }
            }
          }
          return reviver.call(holder, key, value);
        })({"": result}, "")
      : result;
  };
}