JSON.parse()

JSON.parse() 方法用来解析 JSON 字符串,构造由字符串描述的 JavaScript 值或对象。提供可选的 reviver 函数用以在返回之前对所得到的对象执行变换 (操作)。

尝试一下

语法

JSON.parse(text[, reviver])

参数

text

要被解析成 JavaScript 值的字符串,关于 JSON 的语法格式,请参考:JSON

reviver 可选

转换器,如果传入该参数 (函数),可以用来修改解析生成的原始值,调用时机在 parse 函数返回之前。

返回值

Object 类型,对应给定 JSON 文本的对象/值。

异常

若传入的字符串不符合 JSON 规范,则会抛出 SyntaxError 异常。

示例

使用 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

使用 reviver 函数

如果指定了 reviver 函数,则解析出的 JavaScript 值(解析值)会经过一次转换后才将被最终返回(返回值)。更具体点讲就是:解析值本身以及它所包含的所有属性,会按照一定的顺序(从最最里层的属性开始,一级级往外,最终到达顶层,也就是解析值本身)分别的去调用 reviver 函数,在调用过程中,当前属性所属的对象会作为 this 值,当前属性名和属性值会分别作为第一个和第二个参数传入 reviver 中。如果 reviver 返回 undefined,则当前属性会从所属对象中删除,如果返回了其他值,则返回的值会成为当前属性新的属性值。

当遍历到最顶层的值(解析值)时,传入 reviver 函数的参数会是空字符串 ""(因为此时已经没有真正的属性)和当前的解析值(有可能已经被修改过了),当前的 this 值会是 {"": 修改过的解析值},在编写 reviver 函数时,要注意到这个特例。(这个函数的遍历顺序依照:从最内层开始,按照层级顺序,依次向外遍历)

js

JSON.parse('{"p": 5}', function (k, v) {
  if (k === "") return v; // 如果到了最顶层,则直接返回属性值,
  return v * 2; // 否则将属性值变为原来的 2 倍。
}); // { p: 10 }

JSON.parse('{"1": 1, "2": 2,"3": {"4": 4, "5": {"6": 6}}}', function (k, v) {
  console.log(k); // 输出当前的属性名,从而得知遍历顺序是从内向外的,
  // 最后一个属性名会是个空字符串。
  return v; // 返回原始属性值,相当于没有传递 reviver 参数。
});

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

JSON.parse() 不允许用逗号作为结尾

js

// both will throw a SyntaxError
JSON.parse("[1, 2, 3, 4, ]");
JSON.parse('{"foo" : 1, }');

Polyfill

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 parseable, then a SyntaxError is thrown.

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

规范

Specification
ECMAScript Language Specification
# sec-json.parse

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