【前端三分钟】写一个JSON.Parse解析器

//使用递归降序法
let JsonParse = function() {
    let at, //当前字符索引
      ch, //当前字符
      escapee = {
        '"': '"',
        '\\': '\\',
        '/': '/',
        b: 'b',
        f: '\f',
        n: '\n',
        r: '\r',
        t: '\t'
      },
      text,
      error = function (m) {
      //错误处理
        throw {
          name: 'SyntaxError',
          message: m,
          at: at,
          text: text
        };
      },
      next = function (c) {
        //检验c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 = function() {
        //解析一个数字值
        let 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 = function () {
        //解析一个字符串
        var hex,
          i,
          string = '',
          uffff;
        //当解析字符串值时，必须找到"和\字符
        if (ch==='"') {
          while (next()) {
            if (ch === '"') {
              next();
              return string;
            }else if (ch === '\\') {
              next();
              if (ch === 'u') {
                uffff = 0;
                for (i = 0; i < 4;i +=1) {
                  hex = parseInt(next(), 16);
                  if (!isFinite(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 string");
      },
      white = function() {
        //跳过空白
        while (ch && ch <= ' ') {
          next();
        }
      },
      word = function() {
        //true,false或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+"'");
      },
      array = function () {
        //解析一个数组值
        let 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 = function () {
        //解析一个对象值
        let 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 = function () {
        //解析一个JSON值，它可以是对象、数组、字符串、数字或词
        white();
        switch (ch) {
          case '{':
            return object();
          case '[':
            return array();
          case '"':
            return string();
          case '-':
            return number();
          default:
            return ch >= '0' && ch<='9' ? number() : word();
        }
      };
      //返回JsonParse函数
      return function (source,reviver) {
        let result;
        text= source;
        at = 0;
        ch = ' ';
        result = value();
        white();
        if (ch) {
          error("Syntax error");
        }
        //如果存在reviver函数，则递归对这个新结构调用walk函数
        //开始时先创建一个临时启动对象，并以一个空字符串作为键名保存结果
        //如果没有reviver函数，就简单返回结果。
        return typeof reviver === 'function' ? 
          function walk(holder, key) {
            var k,v,val = holder[key];
            if(val && typeof val === 'object') {
              for(k in val) {
                if(Object.hasOwnProperty.call(val,k)) {
                  v = walk(val,k);
                  if(v !== undefined) {
                    val[k] = v;
                  }else {
                    delete val[k];
                  }
                }
              }
            }
            return reviver.call(holder,key,val);
          }({'': result},'') : result;
      }
  }();