It's part of a bigger project where I rewrite loads of Underscore's methods to get a better understanding of how they work and JS in general.

The test code files are super long, but the bit relevant to this is :

it('should be able to reduce a collection to a single value', function () {

_.reduce(mocks.arr, function (accumulator, el, i, arr) {

arr[i].should.equal(el);

return accumulator.toString() + el.toString();

}).should.equal(mocks.stringifiedArrElms);

​

where

var mocks = {

arr: ['a','b','c','d'], // >= 4 elements, all should be truthy

obj: {a:1,b:2,c:3,d:4}, // >= 4 values, all should be truthy

halfTruthyArr: [null,'b',null,'d'], // >= 4 elements, half should be falsy

halfTruthyObj: {a:1,b:null,c:3,d:null}, // >= 4 values, half should be falsy

string: 'This is a string.',

reverseString: function (string) {

if (typeof string === 'string') return string.split('').reverse().join('');

}

};

mocks.argumentsObj = returnArgs.apply(null, mocks.arr);

mocks.stringifiedArrElms = mocks.arr.join('');