A simple implementation of JSON.stringify; covers every case I could think of

stringify.js

function stringify(obj) {
    if (typeof obj !== 'object' || obj === null || obj instanceof Array) {
        return value(obj);
    }

    return '{' + Object.keys(obj).map(function (k) {
        return (typeof obj[k] === 'function') ? null : '"' + k + '":' + value(obj[k]);
    }).filter(function (i) { return i; }) + '}';
}

function value(val) {
    switch(typeof val) {
        case 'string':
            return '"' + val.replace(/\\/g, '\\\\').replace('"', '\\"') + '"';
        case 'number': 
        case 'boolean':
            return '' + val;
        case 'function':
            return 'null';
        case 'object':
            if (val instanceof Date)  return '"' + val.toISOString() + '"';
            if (val instanceof Array) return '[' + val.map(value).join(',') + ']';
            if (val === null)         return 'null';
                                      return stringify(val);
    }
}

var assert = require('assert');

describe('stringify', function () {
    function check(o) {
        return function () {
            assert.equal(stringify(o), JSON.stringify(o));
        };
    }

    it("string", check('andrew'));
    it('string with special chars', check('this"is a \\test'));
    it("number", check(10));
    it("true", check(true));
    it("false", check(false));
    it("null", check(null));
    it("array", check(['one', 'two', 1, { name: 'andrew'}]));
    it("empty object", check({}));
    it("string prop", check({ name: "andrew" }));
    it("number prop", check({ name: "andrew", age: 24 }));
    it("boolean prop", check({ name: "andrew", age: 24, married: false, single: true }));
    it("date prop", check({ name: "andrew", age: 24, married: false, single: true, date: new Date() }));
    it("array prop of strings", check({ array: ['one', 'two'] }));
    it("array prop of differing values", check({ array: ['one', 2, false, null, { value: 'five', or: 2}] }));
    it("null prop", check({ array: ['one', 'two'], nothing: null }));
    it("object prop", check({ name: 'andrew', address: { streetAddress: '21st street', city: 'New York', state: 'NY'}}));
    it("functions", check({ name: 'andrew', doSomething: function () {} }));
    it("functions in array property", check({ name: 'andrew', doSomething: [function () {}] }));
});

@PoojaSSW @ghost I think what's happening there is if null is passed in, it'll get into value which will do typeof null which actually returns object and then within that case we ultimately return the string 'null', which is the same result as you'd get if you called the native JSON.stringify(null). Note also, that the check test method is actually comparing this custom stringify to the native one for all cases (with the obvious assumption that if the results are equal, the custom one is correct). Another approach I've seen, is to wrap the custom stringify in JSON.parse and ensure it's deserialized back to the original raw JavaScript which also seems like a good approach.

In line 2 obj === null will return false if obj is null.

I don't see how since if you do null === null console is saying true

It looks like this will fail for arrays with undefined values and will omit that indice entirely when it should be replaced with null. For example, it looks like functions ARE handled properly but not undefined in this output:

stringify([1, fn, 2])
"[1,null,2]"
stringify([1, undefined, 2])
"[1,,2]"

Might actually be a symptom of undefined not being handled properly overall. I'm coding this up for practice myself and I have this guard at the top of my recursive function:

if (o === undefined) return undefined

I suppose you'd do this similar to how you're already handling null

what does the string regex handler mean?
Line 14:

return '"' + val.replace(/\\/g, '\\\\').replace('"', '\\"') + '"';

it replaces the single-backslash into triple-backslashes('\'), then make the "" to be \\", any use cases?
For example:

const s = "hello, \"alice\"";

Json.stringnify output :

"\"hello, \\\"alice\\\"\""

Your output:

"\"hello, \\\"alice\"\""

Just want to know how and where do you know the rules of JSON.stringnify does?

I have created the whole implementation of the JSON.stringify() method in a recursive way. Here is the link:
https://javascript.plainenglish.io/create-your-own-implementation-of-json-stringify-simiplied-version-8ab6746cdd1

Github link: https://github.com/siddharth-sunchu/native-methods/blob/master/JSONStringfy.js