Defaultdict equivalent in javascript

Question

In python you can have a defaultdict(int) which stores int as values. And if you try to do a 'get' on a key which is not present in the dictionary you get zero as default value.

Can you do the same in javascript/jquery

Answer 1

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) // [] 

Answer 2

Check out pycollections.js:

var collections = require('pycollections');  var dd = new collections.DefaultDict(function(){return 0}); console.log(dd.get('missing'));  // 0  dd.setOneNewValue(987, function(currentValue) {   return currentValue + 1; });  console.log(dd.items()); // [[987, 1], ['missing', 0]]