Defaultdict equivalent in javascript

You can build one using a JavaScript Proxy

var defaultDict = new Proxy({}, {
  get: (target, name) => name in target ? target[name] : 0
})

This lets you use the same syntax as normal objects when accessing properties.

defaultDict.a = 1
console.log(defaultDict.a) // 1
console.log(defaultDict.b) // 0

To clean it up a bit, you can wrap this in a constructor function, or perhaps use the class syntax.

class DefaultDict {
  constructor(defaultVal) {
    return new Proxy({}, {
      get: (target, name) => name in target ? target[name] : defaultVal
    })
  }
}

const counts = new DefaultDict(0)
console.log(counts.c) // 0

EDIT: The above implementation only works well with primitives. It should handle objects too by taking a constructor function for the default value. Here is an implementation that should work with primitives and constructor functions alike.

class DefaultDict {
  constructor(defaultInit) {
    return new Proxy({}, {
      get: (target, name) => name in target ?
        target[name] :
        (target[name] = typeof defaultInit === 'function' ?
          new defaultInit().valueOf() :
          defaultInit)
    })
  }
}


const counts = new DefaultDict(Number)
counts.c++
console.log(counts.c) // 1

const lists = new DefaultDict(Array)
lists.men.push('bob')
lists.women.push('alice')
console.log(lists.men) // ['bob']
console.log(lists.women) // ['alice']
console.log(lists.nonbinary) // []