Mastering JavaScript

Data Structures and Manipulation
By default, range() populates the array with integers, but with a little trick, you can
populate other data types also:
console.log(_.range(3).map(function () { return 'a' }) );
[ 'a', 'a', 'a' ]
This is a fast and convenient way to create and initialize an array with values.
We frequently do this by traditional loops.
The map() function produces a new array of values by mapping each value in
the list through a transformation function. Consider the following example:
var _ = require('underscore');
console.log(_.map([1, 2, 3], function(num){ return num * 3; }));
//[3,6,9]
The reduce() function reduces a list of values to a single value. The initial state is
passed by the iteratee function and each successive step is returned by the iteratee.
The following example shows the usage:
var _ = require('underscore');
var sum = _.reduce([1, 2, 3], function(memo, num){
console.log(memo,num);return memo + num; }, 0);
console.log(sum);
In this example, the line, console.log(memo,num);, is just to make the idea clear.
The output will be as follows:
0 1
1 2
3 3
6
The final output is a sum of 1+2+3=6. As you can see, two values are passed to the
iteratee function. On the first iteration, we call the iteratee function with two values
(0,1)—the value of the memo is defaulted in the call to the reduce() function and
1 is the first element of the list. In the function, we sum memo and num and return
the intermediate sum, which will be used by the iterate() function as a memo
parameter—eventually, the memo will have the accumulated sum. This concept is
important to understand how the intermediate states are used to calculate eventual
results.
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