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7.5.3 Mixins
An object that defines a set of properties that can be used with the tddjs.extend
method to “bless” other objects is often called a mixin . For instance, the Ruby
standard library defines a bunch of useful methods in its Enumerable module,
which may be mixed in to any object that supports the each method. Mixins provide
an incredibly powerful mechanism for sharing behavior between objects. We could
easily port the enumerable module from Ruby to a JavaScript object and mix it in
with, e.g., Array.protoype to give all arrays additional behavior (remember to
not loop arrays with for-in ). Listing 7.58 shows an example that assumes that the
enumerable object contains at least a reject method.
Listing 7.58 Mixing in the enumerable object to Array.prototype
TestCase("EnumerableTest", {
"test should add enumerable methods to arrays":
function () {
tddjs.extend(Array.prototype, enumerable);
var even = [1, 2, 3, 4].reject(function (i) {
return i%2==1;
assertEquals([2, 4], even);
Assuming we are in a browser that supports Array.prototype.forEach ,
we could implement the reject method as seen in Listing 7.59.
Listing 7.59 Excerpt of JavaScript implementation of Ruby's enumerable
var enumerable = {
/* ... */
reject: function (callback) {
var result = [];
this.forEach(function (item) {
if (!callback(item)) {
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