Flatten Object in-place in JavaScript

Question

In a recent interview, I was asked to create a flat method which flattens an object. I tried to implement it like following, but it doesn't work:

Object.defineProperty(Object.prototype, 'flat', {
  enumerable: false,
  writable: true,
  configurable: false,
  value: function() {
      function flatten(obj) {
      for(let key in obj) {
        if(obj.hasOwnProperty(key)) {
          if(typeof obj[key] === 'object') {
            obj[key] = flatten(obj[key])
          }
          else {
           obj[key] = obj[key];
          }
        }
      }
      return obj;
    }
  flatten(this);
  }
});

var obj = { a: 1, b: 2, c: { d: { e: 5 }, f: 6 } }
obj.flat();
console.log(obj); // should return { a: 1, b: 2, e: 5, f: 6 }

What's wrong in my code?

PS: Other post on SO for flattening Object don't do it in-place, which is the requirement in this case.

Answer 1

With help from the other answer, I got the exact code I wanted:

Object.defineProperty(Object.prototype, 'flat', {
  enumerable: false,
  writable: true,
  configurable: false,
  value: function() {
      function flatten(base, obj) {
        for(let key in obj) {
          if(obj.hasOwnProperty(key)) {
            if(typeof obj[key] === 'object' && obj[key] !== null) {
              flatten(base, obj[key]);
              delete obj[key];
            }
            else {
             base[key] = obj[key];
            }
          }
        }
      }
    flatten(this, this);
  }
});

var obj = { a: 1, b: 2, c: { d: { e: 5 }, f: 6 } }
obj.flat();
console.log(obj); // returns { a: 1, b: 2, e: 5, f: 6 }

Answer 2

You're creating the same recursive object structure here:

obj[key] = flatten(obj[key])

You need to put the nested properties onto the original object instead. You also need to remove the intermediate properties from the original object, so that you're left with only the properties at the end.

I'd also highly, highly recommend against mutating built-in prototypes, especially Object.prototype - it can lead to compatibility issues and confusing behavior, especially when libraries are involved. Things will break eventually as a result. Use a plain function instead.

const flatten = (base, obj = base) => {
  for (const [key, value] of Object.entries(obj)) {
    if (typeof value === 'object' && value !== null) {
      delete obj[key];
      flatten(base, value);
    } else {
      base[key] = value;
    }
  }
};
var obj = {
  a: 1,
  b: 2,
  c: {
    d: {
      e: 5
    },
    f: 6
  }
}
flatten(obj);
console.log(obj);