Non-mutating actors for Array and Object in CoffeeScript / JavaScript (like immutable, but not completely, results are still usual Array or Object that you can mutate)

nma.coffee

NMA = 
Array:
  push: (array, items...) ->
    array.concat(items)
  set: (array, index, item) ->
    array[...index].concat([item]).concat(array[index+1...])
  del: (array, index) ->
    (item for item, i in array when i isnt index)

Object:
  set: (obj, key, value) -> 
    result = (NMA.Object.del(obj, key))
    result[key] = value
    result
  del: (obj, key) ->
    result = {}
    result[k] = v for own k, v of obj when k isnt key
    result

nma.js

var NMA,
  slice = [].slice,
  hasProp = {}.hasOwnProperty;

NMA = {
  Array: {
    push: function() {
      var array, items;
      array = arguments[0], items = 2 <= arguments.length ? slice.call(arguments, 1) : [];
      return array.concat(items);
    },
    set: function(array, index, item) {
      return array.slice(0, index).concat([item]).concat(array.slice(index + 1));
    },
    del: function(array, index) {
      var i, item, j, len, results;
      results = [];
      for (i = j = 0, len = array.length; j < len; i = ++j) {
        item = array[i];
        if (i !== index) {
          results.push(item);
        }
      }
      return results;
    }
  },
  Object: {
    set: function(obj, key, value) {
      var result;
      result = NMA.Object.del(obj, key);
      result[key] = value;
      return result;
    },
    del: function(obj, key) {
      var k, result, v;
      result = {};
      for (k in obj) {
        if (!hasProp.call(obj, k)) continue;
        v = obj[k];
        if (k !== key) {
          result[k] = v;
        }
      }
      return result;
    }
  }
};