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)。

语法

JSON.parse(text[, reviver])

参数

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()

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）

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

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

Polyfill

// 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");
    };
}

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