Collections

map

The .map function accepts an array and an iteratee function, the iteratee produces a transformed copy of each array object.

The iteratee function provides 3 arguments

1. item - The current iterated object
2. i - The index of the iterated object
3. list - A reference to the original array/list

The new Array will be of the same length as the old array but will hold the trasformed objects

Example:

_.map([1, 2, 3, 4], function(item, i, list) {
    return (item*item);
});
// [1, 4, 9, 16]

A more concise way to write the above example using ES6 would be

_.map([1, 2, 3, 4], (item, i, list) => {
    return (item*item);
});

or using an inline lambda expression

_.map([1, 2, 3, 4], (item, i, list) => (item*item));

Map is also useful when you want to pluck properties from objects and make an array of them

Example:

let people = [{name: 'he-man', age: 22}, {name: 'man-at-arms', age: 44}];

_.map(people, function(item) {
    return item.name;
});
// ['he-man', 'man-at-arms']

each

The _.each function accepts an array or an object, an iteratee function and an optional context object, the iteratee function is invoked once and in order for each array item The iteratee function provides 3 arguments

item - The current iterated object (or value if an object was passed)
i - The index of the iterated object (or key if an object was passed)
list - A reference to the original array/list (the object that was passed)

Example 1:

_.each(["hello", "underscore"], function(item, i, list) {
    alert(item)
});

The above example will show 2 alerts, the first with the words “hello” and the second for “world”.

Example 2:

_.each({one: 1, two: 2, three: 3}, (value, key, object) => 
    console.log(JSON.stringify(object));
);

This example will log a stringified version of the object 3 times.

.each is a terminal operation, and unlike other intermediate functions (map, pluck, values etc..) you don’t need to return inside the iteratee function body.