pythonic Nested default dictionary in javascript

Question

I want to implement a nested default dictionary in javascript, so I can go

nest['first']['second']['third'] = 'yay' without having to check or initialize nest['first'] & nest['first']['second']

In python, this is pretty easy

nested = defaultDict(lambda: deafultDict(dict()))

I tried to implement the same in javascript using Proxy. The results were not as expected and I can't figure out what I'm missing. From the last logline, it looks like the same default dictionary is being passed around as a reference. How can I avoid this and make it work as expected?

function defaultDict (defaultValue) {
  const handler = {
    get: (obj, prop) => {
      return prop in obj ? obj[prop] : defaultValue
    }
  }
  return new Proxy({}, handler)
}

var nested = defaultDict(defaultDict({}))

nested['first']['second']['third'] = 'yay'

console.log(nested['first']['second']['third']) // yay  OK
console.log(nested['first']['second']) // {third: 'yay'} OK

console.log(nested['first']) // result: {} expected {second: {third: 'yay'}}

console.log(nested['none']['gggggggggggg']) // result: {third: 'yay'} expected: {}

thanks!

Well, yes, your innermost {} in defaultDict(defaultDict({})) is a single plain object. — AKX
– AKX, Commented Sep 18, 2020 at 11:03
sure, but the last logline is printing {third: 'yay'} and not {} as expected — Michael P
– Michael P, Commented Sep 18, 2020 at 11:04

AKX · Accepted Answer · 2020-09-18 11:35:52Z

2

To achieve an arbitrarily deep autovivifying object, you'll want something like

function defaultDict(defaultValueFn) {
  const handler = {
    get: (obj, prop) => {
      if(!(prop in obj)) obj[prop] = defaultValueFn();
      return obj[prop];
    },  
  };
  return new Proxy({}, handler);
}

var nestedDefaultDict = () => defaultDict(nestedDefaultDict);
var nested = nestedDefaultDict();

nested.a.b.c = 8;
nested.d.e.f = 8;
console.log(nested); // { a: { b: { c: 8 } }, d: { e: { f: 8 } } }

, that is just like in the Python defaultdict, you pass in a callable that returns the nested object, not the object itself.

edited Sep 18, 2020 at 11:35

answered Sep 18, 2020 at 11:05

AKX

171k16 gold badges146 silver badges228 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Michael P Over a year ago

this does not give me the expected results unfortunately jsfiddle.net/6k8m1wth

AKX Over a year ago

Sorry, fixed. I missed that we didn't actually insert the new object into the proxied object.

Collectives™ on Stack Overflow

pythonic Nested default dictionary in javascript

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related