_.iteratee

iteratee_.iteratee(value, [context])
Generates a callback that can be applied to each element in a collection. _.iteratee supports a number of shorthand syntaxes for common callback use cases. Depending upon value's type, _.iteratee will return:

// No value
_.iteratee();
=> _.identity()

// Function
_.iteratee(function(n) { return n * 2; });
=> function(n) { return n * 2; }

// Object
_.iteratee({firstName: 'Chelsea'});
=> _.matcher({firstName: 'Chelsea'});

// Anything else
_.iteratee('firstName');
=> _.property('firstName');

The following Underscore methods transform their predicates through _.iteratee: countBy, every, filter, find, findIndex, findKey, findLastIndex, groupBy, indexBy, map, mapObject, max, min, partition, reject, some, sortBy, sortedIndex, and uniq