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 () {}] }));
});

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