Serializing object that contains cyclic object value

Use the second parameter of stringify, the replacer function, to exclude already serialized objects:

var seen = [];JSON.stringify(obj, function(key, val) {   if (val != null && typeof val == "object") {        if (seen.indexOf(val) >= 0) {            return;        }        seen.push(val);    }    return val;});

http://jsfiddle.net/mH6cJ/38/

As correctly pointed out in other comments, this code removes every "seen" object, not only "recursive" ones.

For example, for:

a = {x:1};obj = [a, a];

the result will be incorrect. If your structure is like this, you might want to use Crockford's decycle or this (simpler) function which just replaces recursive references with nulls:

function decycle(obj, stack = []) {    if (!obj || typeof obj !== 'object')        return obj;        if (stack.includes(obj))        return null;    let s = stack.concat([obj]);    return Array.isArray(obj)        ? obj.map(x => decycle(x, s))        : Object.fromEntries(            Object.entries(obj)                .map(([k, v]) => [k, decycle(v, s)]));}//let a = {b: [1, 2, 3]}a.b.push(a);console.log(JSON.stringify(decycle(a)))