Default Map

A map that gives you a default value if a key is not present. Requires an ES6 runtime. Can be subclassed.

var withArray = new DefaultMap([], (map, k) => {
  const xs = []
  map.set(k, xs);
  return xs;
})
 
withArray.get("someKey").push(1);
withArray.get("someKey") // [1]
 
// instances of 'MapWithArrays' would perform like `withArray` above
class MapWithArrays extends DefaultMap {
  constructor(members) {
    super(members, (m, k, set) => {
      set([])
    })
  }
}
 
 
class Counter extends DefaultMap {
  constructor(members) {
    super(members, () => 0)
  }
  add(k, n) {
    const now = this.get(k) + n;
    this.set(k, now)
    return now;
  }
}
 
var visits = new Counter;
visits.get("newUser") // 0, not mutating visits
 
visits.add("newUser", 10)
visits.get("newUser") // 10
 

You control the default operation by providing a function that returns the default value for a key. It's your responsibility to store it, if you'd like. If your default is a mutable value like an array storing it makes sense. If it's a primitive like a number, it's likely better to save space and simply return it.

API

DefaultMap(members: enumerable (as per Map), default: (m: Map, k: any, setAndReturn: (v: any) => void) => defaultValue)

Simply pass in a function to create the default value. It will be called with the map and a key that was requested but not found. You can mutate the map if you like. Whatever you return will be returned from .get(k).

Since it's a pretty common pattern to want to mutate and return the value, you may call the third argument, setAndReturn. The value passed with be the new value of .get(k), and will be returned. The return value of default will be ignored if setAndReturn is called.

// example of using setAndReturn
var withArray = new DefaultMap([], (_m, _k, set) => set([]))
 
withArray.get("someKey").push(1);
withArray.get("someKey") // [1]

Install & use

npm i -S defaultmap

var DefaultMap = require("defaultmap");