JSON is a simple lightweight serialization format that is much used by Frameworks. Atlas, Live, YUI are heavy users.

A JSON global object is added. It provides parse and stringify methods.

JSON.parse(source, reviver)

This method parses a JSON text to produce an object or array. It can throw a SyntaxError exception.

The optional reviver parameter is a function that can filter and transform the results. It receives each of the keys and values, and its return value is used instead of the original value. If it returns what it received, then the structure is not modified. If it returns undefined then the member is deleted.

Example:

Parse the text. Values that look like ISO date strings will be converted to Date objects.

  myData = JSON.parse(text, function (key, value) {
      var a;
      if (typeof value === 'string') {
          a =
  /^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2}):(\d{2}(?:\.\d*)?)Z$/.exec(value);
          if (a) {
              return new Date(Date.UTC(+a[1], +a[2] - 1, +a[3], +a[4],
                  +a[5], +a[6]));
          }
      }
      return value;
  });
  

Parse the text. String values that look like “Date(—date—)” are turned into Date objects.

  
  myData = JSON.parse('["Date(09/09/2001)"]', function (key, value) {
      var d;
      if (typeof value === 'string' &&
              value.slice(0, 5) === 'Date(' &&
              value.slice(-1) === ')') {
          d = new Date(value.slice(5, -1));
          if (d) {
              return d;
          }
      }
      return value;
  });

This implementation is informative:

  JSON.parse = function () {

This is a function that can parse a JSON text, producing a JavaScript data structure. It is a simple, recursive descent parser.

We are defining the function inside of another function to avoid creating global variables.

      var at,     // The index of the current character
          ch,     // The current character
          escapee = {
              '"':  '"',
              '\\': '\\',
              '/':  '/',
              b:    '\b',
              f:    '\f',
              n:    '\n',
              r:    '\r',
              t:    '\t'
          },
          text,
  
          error = function (m) {

Call error when something is wrong.

              throw {
                  name:    'SyntaxError',
                  message: m,
                  at:      at,
                  text:    text
              };
          },
  
          next = function (c) {

If a c parameter is provided, verify that it matches the current character.

              if (c && c !== ch) {
                  error("Expected '" + c + "' instead of '" + ch + "'");
              }

Get the next character. When there are no more characters, return the empty string.

              ch = text.charAt(at);
              at += 1;
              return ch;
          },
  
          number = function () {
  

Parse a number value.

              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 = function () {

Parse a string value.

              var hex,
                  i,
                  string = "",
                  uffff;

When parsing for string values, we must look for " and \ characters.

              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 () {

Skip whitespace.

              while (ch && ch <= ' ') {
                  next();
              }
          },
  
          word = function () {

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,  // Place holder for the value function.
  
          array = function () {

Parse an array value.

              var array = [];
  
              if (ch === '[') {
                  next('[');
                  white();
                  if (ch === ']') {
                      next(']');
                      return array;   // empty array
                  }
                  while (ch) {
                      array.push(value());
                      white();
                      if (ch === ']') {
                          next(']');
                          return array;
                      }
                      next(',');
                      white();
                  }
              }
              error("Bad array");
          },
  
          object = function () {

Parse an object value.

              var key,
                  object = {};
  
              if (ch === '{') {
                  next('{');
                  white();
                  if (ch === '}') {
                      next('}');
                      return object;   // empty object
                  }
                  while (ch) {
                      key = string();
                      white();
                      next(':');
                      object[key] = value();
                      white();
                      if (ch === '}') {
                          next('}');
                          return object;
                      }
                      next(',');
                      white();
                  }
              }
              error("Bad object");
          };
  
      value = function () {

Parse a JSON value. It could be an object, an array, a string, a number, or a word.

          white();
          switch (ch) {
          case '{':
              return object();
          case '[':
              return array();
          case '"':
              return string();
          case '-':
              return number();
          default:
              return ch >= '0' && ch <= '9' ? number() : word();
          }
      };

Return the JSON.parse function. It will have access to all of the above functions and variables.

      return function parse(source, reviver) {
          var result;
  
          text = source;
          at = 0;
          ch = ' ';
          result = value();
          white();
          if (ch) {
              error("Syntax error");
          }

If there is a reviver function, we recursively walk the new structure, passing each name/value pair to the reviver function for possible transformation, starting with a temporary boot object that holds the result in an empty key. If there is not a reviver function, we simply return the result.

          return typeof reviver === 'function' ? function walk(holder, key) {
              var k, v, value = holder[key];
              if (value && typeof value === 'object') {
                  for (k 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;
      };
  }();

JSON.stringify(value, replacer, space)

  value       any JavaScript value, usually an object or array.
  replacer    an optional parameter that determines how object
              values are stringified for objects without a toJSON
              method. It can be a function or an array.
  space       an optional parameter that specifies the indentation
              of nested structures. If it is omitted, the text will
              be packed without extra whitespace. If it is a number,
              it will specify the number of spaces to indent at each
              level. If it is a string (such as '\t' or '&nbsp;'),
              it contains the characters used to indent at each level.

The stringify method produces a JSON text from a JavaScript value. If value is an object or array, the structure will be visited recursively to determine the serialization of each membr or element. The structure must not be cyclical.

When an object value is found, if the object contains a toJSON method, its toJSON method will be called and the result will be stringified. A toJSON method does not serialize: it returns the value represented by the name/value pair that should be serialized, or undefined if nothing should be serialized. The toJSON method will be passed the key associated with the value, and this will be bound to the object holding the key.

For example, this would serialize Dates as ISO strings.

      Date.prototype.toJSON = function (key) {
          return this.toISOString();
      };

You can provide an optional replacer method. It will be passed the key and value of each member, with this bound to the containing object. The value that is returned from your method will be serialized. If your method returns undefined, then the member will be excluded from the serialization.

If the replacer parameter is an array, then it will be used to select the members to be serialized. It filters the results such that only members with keys listed in the replacer array are stringified.

Values that do not have JSON representations, such as undefined or functions, will not be serialized. Such values in objects will be dropped; in arrays they will be replaced with null. You can use a replacer function to replace those with JSON values. JSON.stringify(undefined) returns undefined.

The optional space parameter produces a stringification of the value that is filled with line breaks and indentation to make it easier to read.

If the space parameter is a non-empty string, then that string will be used for indentation. If the space parameter is a number, then the indentation will be that many spaces.

Example:

  text = JSON.stringify(['e', {pluribus: 'unum'}]);
  // text is '["e",{"pluribus":"unum"}]'
  text = JSON.stringify(['e', {pluribus: 'unum'}], null, '\t');
  // text is '[\n\t"e",\n\t{\n\t\t"pluribus": "unum"\n\t}\n]'
  text = JSON.stringify([new Date()], function (key, value) {
      return this[key] instanceof Date ?
          'Date(' + this[key] + ')' : value;
  });
  // text is '["Date(---current time---)"]'

This implementation is informative:

JSON.stringify = function () {

  var gap,
      indent,
      rep;
  function str(key, holder) {

Produce a string from holder[key].

      var i,          // The loop counter.
          k,          // The member key.
          v,          // The member value.
          length,
          mind = gap,
          partial,
          value = holder[key];

If the value has a toJSON method, call it to obtain a replacement value.

      if (value && typeof value === 'object' &&
              typeof value.toJSON === 'function') {
          value = value.toJSON(key);
      }

If we were called with a replacer function, then call the replacer to obtain a replacement value.

      if (typeof rep === 'function') {
          value = rep.call(holder, key, value);
      }

What happens next depends on the value’s type.

      switch (typeof value) {
      case 'string':
          return value.quote();
      case 'number':

JSON numbers must be finite. Encode non-finite numbers as null.

          return isFinite(value) ? String(value) : 'null';
      case 'boolean':
      case 'null':

If the value is a boolean or null, convert it to a string. Note: typeof null does not produce ‘null’. The case is included here in the remote chance that this gets fixed someday.

          return String(value);

If the type is ‘object’, we might be dealing with an object or an array or null.

      case 'object':

typeof null is ‘object’, so watch out for that case.

          if (!value) {
              return 'null';
          }

Make an array to hold the partial results of stringifying this object value.

          gap += indent;
          partial = [];
          if (Array.isArray(value)) {

The object is an array. Stringify every element. Use null as a placeholder for non-JSON values.

              length = value.length;
              for (i = 0; i < length; i += 1) {
                  partial[i] = str(i, value) || 'null';
              }

Join all of the elements together, separated with commas, and wrap them in brackets.

              v = partial.length === 0 ? '[]' :
                  gap ? '[\n' + gap +
                          partial.join(',\n' + gap) + '\n' +
                              mind + ']' :
                        '[' + partial.join(',') + ']';
              gap = mind;
              return v;
          }

If the replacer is an array, use it to select the members to be stringified.

          if (rep && typeof rep === 'object') {
              length = rep.length;
              for (i = 0; i < length; i += 1) {
                  k = rep[i];
                  if (typeof k === 'string') {
                      v = str(k, value, rep);
                      if (v) {
                          partial.push(k.quote() + (gap ? ': ' : ':') + v);
                      }
                  }
              }
          } else {

Otherwise, iterate through all of the keys in the object.

              for (k in value) {
                  if (Object.hasOwnProperty.call(value, k)) {
                      v = str(k, value, rep);
                      if (v) {
                          partial.push(k.quote() + (gap ? ': ' : ':') + v);
                      }
                  }
              }
          }

Join all of the member texts together, separated with commas, and wrap them in braces.

          v = partial.length === 0 ? '{}' :
              gap ? '{\n' + gap +
                      partial.join(',\n' + gap) + '\n' +
                      mind + '}' :
                    '{' + partial.join(',') + '}';
          gap = mind;
          return v;
      }
  }

Return the JSON.stringify method.

  return function stringify(value, replacer, space) {

The stringify method takes a value and an optional replacer, and an optional space parameter, and returns a JSON text. The replacer can be a function that can replace values, or an array of strings that will select the keys. A default replacer method can be provided. Use of the space parameter can produce text that is more easily readable.

      var i;
      gap = '';
      indent = '';
      if (space) {

If the space parameter is a number, make an indent string containing that many spaces.

          switch (typeof space) {
          case 'number':
              for (i = 0; i < space; i += 1) {
                  indent += ' ';
              }
              break;

If the space parameter is a string, it will be used as the indent string.

          case 'string':
              indent = space;
          }
      }

If there is a replacer, it must be a function or an array. Otherwise, throw an error.

      rep = replacer;
      if (replacer && typeof replacer !== 'function' &&
              !Array.isArray(replacer)) {
          throw new Error('JSON.stringify');
      }

Make a fake root object containing our value under the key of empty string. Return the result of stringifying the value.

      return str("", {"": value});
  };

}();

Relevant Goal: 2
Relevant Design Principle: 2a, 2b, 3
Relevant ES4 proposal: JSON encoding and decoding.

Pratap Lakshman 2007/04/15 02:23

 
es3.1/json_support.txt · Last modified: 2008/05/15 23:03 by crock
 
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