JSON.parse()

js

JSON.parse(text);
JSON.parse(text, reviver);

js

// From https://github.com/douglascrockford/JSON-js/blob/master/json2.js
if (typeof JSON.parse !== "function") {
  var rx_one = /^[\],:{}\s]*$/;
  var rx_two = /\\(?:["\\\/bfnrt]|u[0-9a-fA-F]{4})/g;
  var rx_three =
    /"[^"\\\n\r]*"|true|false|null|-?\d+(?:\.\d*)?(?:[eE][+\-]?\d+)?/g;
  var rx_four = /(?:^|:|,)(?:\s*\[)+/g;
  var rx_dangerous =
    /[\u0000\u00ad\u0600-\u0604\u070f\u17b4\u17b5\u200c-\u200f\u2028-\u202f\u2060-\u206f\ufeff\ufff0-\uffff]/g;
  JSON.parse = function (text, reviver) {
    // The parse method takes a text and an optional reviver function, and returns
    // a JavaScript value if the text is a valid JSON text.

    var j;

    function walk(holder, key) {
      // The walk method is used to recursively walk the resulting structure so
      // that modifications can be made.

      var k;
      var v;
      var value = holder[key];
      if (value && typeof value === "object") {
        for (k in value) {
          if (Object.prototype.hasOwnProperty.call(value, k)) {
            v = walk(value, k);
            if (v !== undefined) {
              value[k] = v;
            } else {
              delete value[k];
            }
          }
        }
      }
      return reviver.call(holder, key, value);
    }

    // Parsing happens in four stages. In the first stage, we replace certain
    // Unicode characters with escape sequences. JavaScript handles many characters
    // incorrectly, either silently deleting them, or treating them as line endings.

    text = String(text);
    rx_dangerous.lastIndex = 0;
    if (rx_dangerous.test(text)) {
      text = text.replace(rx_dangerous, function (a) {
        return "\\u" + ("0000" + a.charCodeAt(0).toString(16)).slice(-4);
      });
    }

    // In the second stage, we run the text against regular expressions that look
    // for non-JSON patterns. We are especially concerned with "()" and "new"
    // because they can cause invocation, and "=" because it can cause mutation.
    // But just to be safe, we want to reject all unexpected forms.

    // We split the second stage into 4 regexp operations in order to work around
    // crippling inefficiencies in IE's and Safari's regexp engines. First we
    // replace the JSON backslash pairs with "@" (a non-JSON character). Second, we
    // replace all simple value tokens with "]" characters. Third, we delete all
    // open brackets that follow a colon or comma or that begin the text. Finally,
    // we look to see that the remaining characters are only whitespace or "]" or
    // "," or ":" or "{" or "}". If that is so, then the text is safe for eval.

    if (
      rx_one.test(
        text.replace(rx_two, "@").replace(rx_three, "]").replace(rx_four, ""),
      )
    ) {
      // In the third stage we use the eval function to compile the text into a
      // JavaScript structure. The "{" operator is subject to a syntactic ambiguity
      // in JavaScript: it can begin a block or an object literal. We wrap the text
      // in parens to eliminate the ambiguity.

      j = eval("(" + text + ")");

      // In the optional fourth stage, we recursively walk the new structure, passing
      // each name/value pair to a reviver function for possible transformation.

      return typeof reviver === "function"
        ? walk(
            {
              "": j,
            },
            "",
          )
        : j;
    }

    // If the text is not JSON parsable, then a SyntaxError is thrown.

    throw new SyntaxError("JSON.parse");
  };
}

js

JSON.parse("{}"); // {}
JSON.parse("true"); // true
JSON.parse('"foo"'); // "foo"
JSON.parse('[1, 5, "false"]'); // [1, 5, "false"]
JSON.parse("null"); // null

js

JSON.parse(
  '{"p": 5}',
  (key, value) =>
    typeof value === "number"
      ? value * 2 // 数値ならば値の2倍を返す
      : value, // それ以外ならば変更しない
);

// { p: 10 }

JSON.parse('{"1": 1, "2": 2, "3": {"4": 4, "5": {"6": 6}}}', (key, value) => {
  console.log(key); // 現在のプロパティ名を出力する。最後は ""。
  return value; // 変更されていないプロパティの値を返す。
});

// 1
// 2
// 4
// 6
// 5
// 3
// ""

js

// 両方とも SyntaxError が発生
JSON.parse("[1, 2, 3, 4, ]");
JSON.parse('{"foo" : 1, }');

js

// SyntaxError が発生
JSON.parse("{'foo': 1}");