let obj = {

"a": {

"b": {

"c": 12,

"d": "Hello World",

"e": null

},

"f": [1,2,3]

}

};

const flatten = (obj, parentkey) => {

return Object.keys(obj).reduce((acc, key) => {

if (Object.prototype.hasOwnProperty.call(obj, key)) {

let val = obj[key];

if (typeof val === "object" && !Array.isArray(val) && val != null) { // != will catch undefined or null

let flat = flatten(val, key);

if (parentkey) {

for (let i in flat) {

let k = `${parentkey}/${i}`;

acc[k] = flat[i];

}

} else {

acc = flat;

}

} else {

let prop = parentkey ? `${parentkey}/${key}` : key;

acc[prop] = val;

}

}

return acc;

}, {});

};

console.log(flatten(obj));

// {

// a/b/c: 12,

// a/b/d: "Hello World",

// a/b/e: null,

// a/f: [1, 2, 3]

// }