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Viren B
Viren B

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Solving "Wherefore art thou" / freeCodeCamp Algorithm Challenges

'Wherefore art thou'

Let's solve 'Wherefore art thou' another Intermediate JavaScript Algorithm Scripting Challenge, from freeCodeCamp. Here is what we are working with:


function whatIsInAName(collection, source) {
  var arr = [];
  // Only change code below this line

  // Only change code above this line
  return arr;

whatIsInAName([{ first: "Romeo", last: "Montague" }, { first: "Mercutio", last: null }, { first: "Tybalt", last: "Capulet" }], { last: "Capulet" });


Make a function that looks through an array of objects (first argument) and returns an array of all objects that have matching name and value pairs (second argument). Each name and value pair of the source object has to be present in the object from the collection if it is to be included in the returned array.

For example, if the first argument is [{ first: "Romeo", last: "Montague" }, { first: "Mercutio", last: null }, { first: "Tybalt", last: "Capulet" }], and the second argument is { last: "Capulet" }, then you must return the third object from the array (the first argument), because it contains the name and its value, that was passed on as the second argument.


whatIsInAName([{ first: "Romeo", last: "Montague" }, { first: "Mercutio", last: null }, { first: "Tybalt", last: "Capulet" }], { last: "Capulet" }) should return [{ first: "Tybalt", last: "Capulet" }].
whatIsInAName([{ "apple": 1 }, { "apple": 1 }, { "apple": 1, "bat": 2 }], { "apple": 1 }) should return [{ "apple": 1 }, { "apple": 1 }, { "apple": 1, "bat": 2 }].
whatIsInAName([{ "apple": 1, "bat": 2 }, { "bat": 2 }, { "apple": 1, "bat": 2, "cookie": 2 }], { "apple": 1, "bat": 2 }) should return [{ "apple": 1, "bat": 2 }, { "apple": 1, "bat": 2, "cookie": 2 }].
whatIsInAName([{ "apple": 1, "bat": 2 }, { "apple": 1 }, { "apple": 1, "bat": 2, "cookie": 2 }], { "apple": 1, "cookie": 2 }) should return [{ "apple": 1, "bat": 2, "cookie": 2 }].
whatIsInAName([{ "apple": 1, "bat": 2 }, { "apple": 1 }, { "apple": 1, "bat": 2, "cookie": 2 }, { "bat":2 }], { "apple": 1, "bat": 2 }) should return [{ "apple": 1, "bat": 2 }, { "apple": 1, "bat": 2, "cookie":2 }].
whatIsInAName([{"a": 1, "b": 2, "c": 3}], {"a": 1, "b": 9999, "c": 3}) should return []

Our Thoughts & Approach

The most important part of solving algorithm challenges is reading. Reading the starter code, the directions, and the tests thoroughly is the approach I take before jumping into coding. If the objective is not understood, I believe it will be a lot harder to code a solution. So what we are working with it is:

  • Our two inputs are an array of objects (collection) and an object (source).

  • We want our ouput to be an array.

  • We need to check if the second argument exists in the first argument (check if the key(s) exists).

Since we are working with objects and arrays in this challenge, let us see if there are any built-in methods we can use to help solve this.

After reading all the information, my first action would be to use a method, Object.keys(), on the second argument, source, to create an array from the keys of the object. I feel it would be easier to check against collection.

Object.keys() on MDN

let source = { "apple": 1, "bat": 2 };
// Array [ "apple", "bat" ]

So we now have an array of objects and an array (of keys to check). The natural instinct (for me) is to now loop through collection, but we want to check if the keys exist and then also check the values.

An array method we've used in the past to check if a test (that we set up) passes is Array.filter(). It will create a new array of the items which pass the test.


One possible issue I can see is the fact we are looping through an array but we need to access an object within the array, not just primitive data types (string, number, etc.).

Reading up more about objects, there is a method for which you can check if the property exists on the object. Example below:

let dog = {name: 'Rusty', age: 4};
// true

Object.hasOwnProperty() on MDN

So, in short, we're going to use arr, an empty array already declared, set that to collection.filter(), loop through collection, use hasOwnProperty() to see if the key exists in the objects within collection, then check if the values are equal, returning the items which are two. Let's break that down a little further:

var arr = []; // This code was provided
var sourceKeys = Object.keys(source) // Creates an array of keys
arr = collection.filter(function(obj) {
  // obj is each item (an object) in collection
  for (var i = 0; i < sourceKeys.length; i++) {
    // we want to loop through based on how many keys they're were in source
    if (!obj.hasOwnProperty(sourceKeys[i]) || obj[sourceKeys[i]] !== source[sourceKeys[i]])   
      // if obj doesnt have the key OR if obj key's value doesn't equal source key's value
      // return false so it get's filtered out
      return false;
  return true;
  // if it passed the above if statement, doesn't get filtered out and goes into arr
return arr;
// return an array

I didn't use pseudocode this time but I hope the comments were helpful.

Our Solution

function whatIsInAName(collection, source) {
  var arr = [];
  // Only change code below this line
  var sourceKeys = Object.keys(source);
  arr = collection.filter(function(obj) {
    for (var i = 0; i < sourceKeys.length; i++) {
      if (!obj.hasOwnProperty(sourceKeys[i]) || obj[sourceKeys[i]] !== source[sourceKeys[i]]) {
        return false;
    return true;
  // Only change code above this line
  return arr;

Resources & Links

'Wherefore art thou' challenge on fCC


Donate to freeCodeCamp

My Solution on GitHub

Thank you for reading!

Top comments (6)

ttatsf profile image
tatsuo fukuchi • Edited

Another way:

const whatIsInAName = (objs, obj) =>
    e => Object.entries(obj).every(
      ([k, v]) => e?.[k] === v
brian87370205 profile image

Hey, quick question .. (bear with me as i'm still learning) ...

I have console.logged this line so many times and it always returns false ...
obj.hasOwnProperty(sourceKeys[i]), and since you have a ! in front of it, it would make it true. It's console logging an array which and not the actual keys in the array. Wouldn't the "obj" need to have a obj[0] in front of it for the hasOwnProperty() method to work ?!
And my second question is, arr = collection.filter(function(obj) .. how does javascript know that the obj value is the array collection ?!

Everything else i understand and thankful for your help!

sourdragon profile image
Adithya Ds

I have the same doubts. Did you figure it out if you did let me know.

soumya98dev profile image
Soumyadeep Chatterjee

Great solution, but I did'not understood, why is the "return true" outside of the for loop. Then after each iteration where are the results of it is stored when the if is true.

kartikongit profile image
kartikongit • Edited

function whatIsInAName(collection, source) {
// "What's in a name? that which we call a rose
// By any other name would smell as sweet.”
// -- by William Shakespeare, Romeo and Juliet
const souceKeys = Object.keys(source);

// filter the collection
return collection.filter(obj => {
for (let i = 0; i < sourceKeys.length; i++) {
if (obj[sourceKeys[i]] !== source[sourceKeys[i]]) {
return false;
return true;

whatIsInAName([{ first: "Romeo", last: "Montague" }, { first: "Mercutio", last: null }, { first: "Tybalt", last: "Capulet" }], { last: "Capulet" });

collection is an array of objects
source is just a single object .. it can contain more than 1 keys

now we stored keys of the source object inside the sourceKeys array

sourceKeys is an array that stores the keys from source

we are returning a filtered array

we are filtering the collection array

we are only using 1 parameter of the callback function for the filter method

this function will be called for each object that resides in collection array

we have the hold of keys from source object

we have the hold of a single object from collection array

now we are going to perform a for loop ..

why for loop ?

what are we looping ?

we are looping through keys ...

and we get hold of the single key from the source key

lets say that key is "last"

now with an if statement we are checking the values of obj[last] !== source[last]

so here comes your question why is return true out of forLoop

well the for LOop is designed to only return false no matter what ... so even if all the key's values matches the for loop will only care about false values

and its job is to see if there is something that is not matching

but since we are using a filter method that actually just takes true values and rejects false values to be incluced

in the end if nothing matches nothing will be added to array

but if it matches the whole object is said to be true and that object will be included

kod14 profile image

Thanks for this post! It helped me understand a lot better of the solution on FCC!