At underscore js, Can I get multiple columns with

Question

var people = [
    {firstName : "Thein", city : "ny", qty : 5},
    {firstName : "Michael", city : "ny", qty : 3},
    {firstName : "Bloom", city : "nj", qty : 10}
];

var results=_.pluck(_.where(people, {city : "ny"}), 'firstName');

For example : I need firstName and qty.

Answer 1

To project to multiple properties, you need map, not pluck:

var results = _.map(
    _.where(people, {city : "ny"}), 
    function(person) {
        return { firstName: person.firstName, qty: person.qty };
    }
);

[{"firstName":"Thein","qty":5},{"firstName":"Michael","qty":3}]

(Fiddle)

Note that, if you wanted to, you could create a helper method "pluckMany" that does the same thing as pluck with variable arguments:

// first argument is the source array, followed by one or more property names
var pluckMany = function() {
    // get the property names to pluck
    var source = arguments[0];
    var propertiesToPluck = _.rest(arguments, 1);
    return _.map(source, function(item) {
        var obj = {};
        _.each(propertiesToPluck, function(property) {
            obj[property] = item[property]; 
        });
        return obj;
    });
};

You can use the _.mixin function to add a "pluckMany" function to the _ namespace. Using this you can write simply:

var results = _.chain(people).where({city : "ny"}).pluckMany( "firstName", "qty").value();

(Fiddle)

Answer 2

We don't have to use pluck, omit do the trick as well.

var result = _.map(people, function(person) {
  return _.omit(person, 'city');
});

Answer 3

TL;DR Use :

var results = _.chain(people)
    .where({ city: "ny" })
    .map(_.partialRight(_.pick, 'firstName', 'qty'))
    .value();

But please read on for explanations as I feel the process of finding this solution is more interesting than the actual answer.

---

The general pattern would be (it works with lodash too) :

_.map(array, function(obj) { return _.pick(obj, 'x', 'y', 'z'); });

Given this general map function which transforms each element of a collection, there are multiple ways to adapt this to your particular situation (that vouch for the flexibility of map, which is a very basic building block of functional programs).

Let me present below several ways to implement our solution :

var _ = require('lodash'); // @lodash 2.4.1 at the time of writing
// use underscore if you want to, but please see http://stackoverflow.com/questions/13789618/differences-between-lodash-and-underscore


/* la data */
var people = [{
    firstName: "Thein",
    city: "ny",
    qty: 5
}, {
    firstName: "Michael",
    city: "ny",
    qty: 3
}, {
    firstName: "Bloom",
    city: "nj",
    qty: 10
}];


/* OPTION1 : mixin' with _ */
_.mixin({
    pluckMany: function() {
        var array = arguments[0],
            propertiesToPluck = _.rest(arguments, 1);

        return _.map(array, function(item) {

            /* Alternative implementation 1.1
             * ------------------------------
             * Taken from @mMcGarnagle answer
             * _each is easy to understand here,
             * but has to modify the variable `obj` from a closure
             * I try to avoid that for trivial cases like this one.
             */
            var obj = {};
            _.each(propertiesToPluck, function(property) {
                obj[property] = item[property];
            });
            return obj;


            /* Alternative implementation 1.2
             * ------------------------------
             * Rewrite the previous code,
             * by passing the accumulator (previously`obj`, but really it is an object that accumulates the result being constructed) across function calls.
             * This construction is typical of the `reduce` function, closer to a functionnal programming style.
             */
            return _.reduce(propertiesToPluck, function(obj, property) {
                obj[property] = item[property];
                return obj;
            }, {});


            /* Alternative implementation 1.3
             * ------------------------------
             * If we are already using lodash/underscore,
             * then let's use the `pick` function ! I also included an example of `flatten` here
             */
            return _.pick(item, _.flatten(propertiesToPluck, true));


            /* Alternative implementation 1.4
             * ------------------------------
             * But really flatten is not needed.
             */
            return _.partial(_.pick, item).apply(null, propertiesToPluck);
        });
    }
});



/* Let's use our mixed function !
 * Since we call several _ functions on the same object
 * it is more readable to chain the calls.
 */
var results = _.chain(people)
    .where({
        city: "ny"
    })
    .pluckMany('firstName', 'qty')
    .value();



/* OPTION 2 : without mixing our code with lodash/underscore */
var results = _.chain(people)
    .where({
        city: "ny"
    })
    .map(_.partialRight(_.pick, 'firstName', 'qty'))
    .value();

console.log(results);

If you like this way of writing code with underscore or lodash, I highly suggest that you have a look at functional programming, as this style of writing as well as many functions (map, reduce amongst many others) come from there.

Note : This is apparently a common question in underscore : https://github.com/jashkenas/underscore/issues/1104

This is apparently no accident if these are left out of underscore/lodash : "composability is better than features". You could also say do one thing and do it well. This is also why _.mixin exists.

Answer 4

Yep, I wish pluck had an option of passing an array, but in the meantime, you could do:

const pluckFields = (arr, fields) => _.map(arr, item => _.pick(item, fields))

Answer 5

YAAUu-Yep Another Answer Using underscore...

// use a proper browser to run this code snippet, a browser that is es6-compliant
let people = [{
    firstName: "Thein",
    city: "ny",
    qty: 5
  },
  {
    firstName: "Michael",
    city: "ny",
    qty: 3
  },
  {
    firstName: "Bloom",
    city: "nj",
    qty: 10
  }
];
// either you pick the properties you want 
let picking = _.iteratee((person) => _(person).pick("firstName", "city"));
// either you omit the properties you do not want
let omitting = _.iteratee((person) => _(person).omit("qty"));
// create the filter by city
let living = (people, city) => _(people).where({
  "city": city
});
// put the "filter by city" into a mixin (as I assume it would be used again & again)
_.mixin({
  living: living
});
// do the thing (twice), 
// these chaining methods could be done into a mixin as well
console.log("results by picking properties:", _(people).chain().living("ny").map(picking).value());
console.log("results by omitting properties:", _(people).chain().living("ny").map(omitting).value());

<script src="https://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.8.3/underscore-min.js"></script>

Answer 6

My understanding is the author of the question wants to take an array of objects with many properties and strip each object down to a small list of properties.

There are myriad ways of doing so with _ , but I like this way best. Pass in an empty result object which will be "this" inside the function. Iterate with _each , and _pick the fields you want:

var myObjects = [
    { "first" : "eric",
      "last" : "gumbo",
      "code" : "x482"
    },
    { "first" : "john",
      "last" : "dinkman",
      "code" : "y9283"
    }
];

var result = [];
_.each( myObjects, function(itm) { this.push(_.pick(itm,"first","code")) }, result );

console.log(result);