mapgist.js

var list1 = [{a: 1}, {b: 2}, {c:3}];
var list2 = _.map(list1, function(i){
    i = "test";
    return "test";
});

//list1 is not changed but a new list is created:
//JSON.stringify(list1):  "[{"a":1},{"b":2},{"c":3}]"
//JSON.stringify(list2);  "["test","test","test"]"


var list3 = _.map(list1, function(i){
    i.a = "hello";
    return "test";
});

//list1 is changed:
//JSON.stringify(list1) "[{"a":"hello"},{"b":2,"a":"hello"},{"c":3,"a":"hello"}]"