tool to run different functions based on the type of an object

README.md

I'm working with Leaflet.js & Leaflet-Editable.js there are a lot places where I need to respond to clicks on, creation of, etc. different types of map features differently. This function allows me to eschew if,else,if,else blocks for these cases.

Example:

Before

if(is(L.Marker, feature)){
  setMarkerStyleActive(feature);
} else if(is(L.Polyline, feature)){
  setPolylineStyleActive(feature);
} else if(is(L.Polygon, feature)){
  setPolygonStyleActive(feature);
} 

After

const setStyleActive = runByType(new Map([
  [L.Marker, setMarkerStyleActive],
  [L.Polyline, setPolylineStyleActive],
  [L.Polygon, setPolygonStyleActive]
]);

//then when I need to use it
setStyleActive(thing);

not super useful if you only have 1 switch like above but with many it becomes useful

example.js

const fnsByType = new Map([
  [String, string => string.toUpperCase()],
  [Number, num => Math.floor(Math.abs(num))]
]);

stringOrNumberChanger = runByType(fnsByType);

console.log(stringOrNumberChanger("Hello"));
console.log(stringOrNumberChanger(-27.673));

function Person(name, age){
  this.name = name;
  this.age = age;
}
Person.prototype.getAge = function(){ return this.age; };

const p = new Person("Sequoia", 101);

fnsByType.set(Person, person => `person is ${person.getAge()} years old`);

//now our map can handle a new type
console.log(stringOrNumberChanger(p));

runByType.js

//from ramda https://github.com/ramda/ramda/blob/master/src/is.js
function is(Ctor, val) {
  return val != null && val.constructor === Ctor || val instanceof Ctor;
}

/**
 * Runs a different function depending on a map of constructors
 * to constructors for those functions
 * 
 * @param {Map<Function, Function>} typeToFnsMap - Constructor, fnToRunOnThatType
 * @return {Function}
 */

export function runByType(typeToFnsMap){
  /**
   * @param {any} thing
   * @return {any}
   */
  return function(thing){
    let funToRun = () => new Error('type not supported');
    typeToFnsMap.forEach(function(fn, Ctor){
      if(is(Ctor, thing)){
        funToRun = fn;
        return;
      }
    });
  
    return funToRun(thing);
  };
}