I recently read a great article by Marko Denic about array methods. Inspired by this, I thought to myself that it might be a great opportunity to explain them thoroughly and show you what you can use them for, and how you could implement them yourselves. Without further ado, let’s get into it.

Table of Contents

1. filter()

filter is the method whenever you want to, well, filter out values. Only want positive values? Only looking for objects that have a certain property filter is your way to go.

The following is the signature of the filter method:

filter(function (element, index, array) {
  // element is the element within the array
  // index is the index of the element in the array
  // array is a reference to the array filter works on
}, thisOverride);
// thisOverride is a way to override the semantical this within the callback function.
// If you set it to another object, calling this.anyThing would access anyThing within that
// object, and not the actual array.

Example Use Case

Imagine that you have an online shop. And now you want to send a discount code to all customers that live in a certain area. YOu could do it like follows:

const getElibigleCustomers(customers, zipCode) {
  return customers.filter(
    (customer) => customer.address.zipCode === zipCode
  );
};

getElibigleCustomers returns all customers that have an address stored with a zipCode, which is the same zipCode you look for. All other customers are filtered out of the array.

Reimplementing filter

If you want to understand filter even better, let’s try to reimplement it:

function filter(callbackFn) {
  const newArray = [];

  for (let i = 0; i < this.length; i++) {
    if (callbackFn(this[i], i, this)) {
      newArray.push(this[i]);
    }
  }

  return newArray;
}

Array.prototype.filter = filter;

Attention: Keep in mind that you should never replace a prototype method of a built-in type yourself. But this is just to show you how a possible implementation might look.

As you see, filter is nothing else than a loop that executes a callback function for each element. All elements the callback function returns false for, are filtered out.

2. forEach()

forEach is a functional way to loop over array elements and execute some logic for each element. The method itself doesn’t return a new array.

The following is the signature of the forEach method:

forEach(function (element, index, array) {
  // element is the element within the array
  // index is the index of the element in the array
  // array is a reference to the array forEach works on
}, thisOverride);
// thisOverride is a way to override the semantical this within the callback function.
// If you set it to another object, calling this.anyThing would access anyThing within that
// object, and not the actual array.

Example Use Case

Let’s stay with the example of the online shop. Now, you want to print all names of the customers you previously filtered out. You could do it like this:

getElibigleCustomers(customers, '123456')
  .forEach(
    (customer) => console.log(`${customer.forename} ${customer.surname}`)
  );

When `forEach“ executes, the console prints the full name of all customers that were previously filtered out.

Reimplementing forEach

Let’s reimplement forEach so you better understand how it works:

function forEach(callbackFn) {
  for (let i = 0; i < this.length; i++) {
    callbackFn(this[i], i, this);
  }
}

Array.prototype.forEach = forEach;

Once again, keep in mind that you should never replace prototype methods of built-in types in a real app, except you really know what you are doing.

You probably begin to notice a pattern by now. forEach is, once again, nothing else than a loop. And within this loop, the callback function is called. The result isn’t interesting and thus thrown away.

3. some()

some is a special array method. It tests whether at least one element within the array tests positive for a specific condition. If so, some returns true, otherwise, it returns false.

The following is the signature of the some method:

some(function (element, index, array) {
  // element is the element within the array
  // index is the index of the element in the array
  // array is a reference to the array some works on
}, thisOverride);
// thisOverride is a way to override the semantical this within the callback function.
// If you set it to another object, calling this.anyThing would access anyThing within that
// object, and not the actual array.

Example Use Case

Back to our online shop example. Imagine that you now want to test whether at least some of the customers you filtered out are underage. If so, you want to show them another deal, because everyone else gets a discount for alcoholic drinks. But you don’t want to support underage children drinking, of course.

This is how you could do it:

const eligibleCustomers = getElibigleCustomers(customers, '123456');

const containsUnderAgedCustomers = eligibleCustomers.some(
  (customer) => customer.age < 18
);

When some executes, it checks each customer’s age property. If at least one is below 18, it returns true.

Reimplementing some

Time to reimplement some:

function some(callbackFn) {
  for (let i = 0; i < this.length; i++) {
    if (callbackFn(this[i], i, this)) {
      return true;
    }
  }
  return false;
}

Array.prototype.some = some;

some loops through all elements of the array until it finds an element the callback function returns true for. In this case, the method returns early, because for some elements to satisfy a condition, one is already enough. Only when no element matches, false is returned.

4. every()

every is the counterpart of some. It tests whether all elements satisfy a condition. Only then, the method returns true. If only one element fails the test, false is returned.

The following is the signature of the every method:

every(function (element, index, array) {
  // element is the element within the array
  // index is the index of the element in the array
  // array is a reference to the array every works on
}, thisOverride);
// thisOverride is a way to override the semantical this within the callback function.
// If you set it to another object, calling this.anyThing would access anyThing within that
// object, and not the actual array.

Example Use Case

Many customers who received your discount code have ordered by now. To save on shipment costs, you want to send it out all at once. But first, you need to check whether all those customers have valid address data saved.

An example implementation could look like this:

const customersWhoOrdered = getCustomersForOrder('discount1234');

const allCustomersHaveValidShipmentData = customersWhoOrdered
  .every(
    (customer) => hasValidShipmentData(customer)
  );

When every executes, it passes each customer to a function that checks whether that customer has valid shipment data stored. If only one customer has invalid data, the whole function returns false.

Reimplementing every

Time to reimplement every:

function every(callbackFn) {
  for (let i = 0; i < this.length; i++) {
    if (!callbackFn(this[i], i, this)) {
      return false;
    }
  }
  return true;
}

Array.prototype.every = every;

every loops through all elements of the array until it finds an element the callback function returns false for. In this case, the method returns early, because not all elements satisfy the condition. One is already enough. Only when no elements match, true is returned.

You might even have noticed that every is not very different from some. At some specific points, true is replaced with false, and the check with the callback function is negated. These small changes are already enough to make the method do exactly what you want.

5. includes()

includes is a method that checks whether an array contains a specific element. It’s good for a quick check, but it also has its drawbacks, which we talk about in a minute.

The following is the signature of the includes method:

includes(function (searchElement, fromIndex) {
  // searchElement is the element you look for
  // fromIndex is the index the search should start at
});

Example Use Case

includes is special. It actually tests for strict equality, which means: It either searches for a value that is strictly equal to the search element, or it looks for the exact reference of an object.

Let’s say you want to check whether the customers orders include a very specific order, like this:

const allOrders = getAllOrders();

const containsSpecificOrder = allOrders.includes({
  customer: 'John Doe'
  zipCode: '54321'
});

You would probably be surprised to find out that include returns false, even if the array contains an object with the exact same properties. This is because it looks for strict equality, and objects are only strictly equal if they are the same reference. JavaScript does not know a deepEquals method.

This reduces includes’ use cases to primitive values. If you, for example want to check whether an array of numbers contains a specific number like this:

const numbers = [1, 2, 3, 4, 5];

const includesFive = numbers.includes(5);

Here is a rule of thumb: If you deal with an array of primitive values, use includes. If you deal with objects, use some because it allows you to pass in a callback with which you can test objects for equality.

Reimplementing includes

Time to reimplement includes:

function includes(searchElement, fromIndex = 0) {
  if (fromIndex > this.length || fromIndex < 0) {
    return false;
  }

  for (let i = fromIndex; i < this.length; i++) {
    if (this[i] === searchElement) {
      return true;
    }
  }

  return false;
}

Array.prototype.includes = includes;

The guard statements at the beginning are there to make the method execution a little safer. A negative fromIndex makes no sense, as well as an index larger than the array’s max index. The rest is just a loop testing each element for strict equality with the element searched for.

6. map()

map is one of the most important array methods out there. Whenever you want to transform all values within an array, map is the way to go.

The following is the signature of the map method:

map(function (element, index, array) {
  // element is the element within the array
  // index is the index of the element in the array
  // array is a reference to the array map works on
}, thisOverride);
// thisOverride is a way to override the semantical this within the callback function.
// If you set it to another object, calling this.anyThing would access anyThing within that
// object, and not the actual array.

Example Use Case

Let’s jump back to the point where you had all eligible customers filtered. Now, you want to send their orders off and need to get all their addresses. This is a great use case for map:

const eligibleCustomers = getElibigleCustomers(customers, '123456');

const addresses = eligibleCustomers
  .map((customer) => customer.address);

map iterates over all customers and then extracts all addresses. These are put into a new array and returned from the function.

Reimplementing map

Time to reimplement map:

function map(callbackFn) {
  const newArray = [];
  for (let i = 0; i < this.length; i++) {
    const mappedValue = callbackFn(this[i], i, this);
    newArray.push(mappedValue);
  }
  return newArray;
}

Array.prototype.map = map;

map iterates over all elements of the array. For each element, it calls the callback function and expects a new value to be returned. This value is then pushed to a new array. That array is returned in full, resulting in an array of the same size as the original one, but with probably different elements in it.

7. reduce()

reduce is the most powerful array method existing. It can be used to reimplement all existing array methods, and it’s the most flexible one. Talking about all the advantages it offers would definitely need an article on its own, but you will get a glimpse of it soon.

The following is the signature of the reduce method:

reduce(function (accumulator, currentValue, currentIndex, array) {
  // accumulator is the result of the last call, or the initialValue in the beginning
  // currentValue is the value currently processed
  // currentIndex is the index of the current value within the array
  // array is a reference to the array reduce works on
}, initialValue);

Example Use Case

Remember when you wanted to find out whether there were underage customers? Another way to deal with this problem is to group all your eligible customers into two groups. Those of legal age, and those who are underage.

This is how you could do it:

const eligibleCustomers = getElibigleCustomers(customers, '123456');

const customerGroups = eligibleCustomers
  .reduce((accumulator, customer) => {
    if (customer.age > 18) {
      accumulator[0].push(customer);
    } else {
      accumulator[1].push(customer);
    }
    return accumulator;
  }, [[], []]);

reduce can be pretty difficult to understand but we can go over the above code together and see what it does. In this case, the accumulator is a 2-dimensional array. Index 0 contains all customers >= 18 years of age. Index 1 contains all customers who are underage. The first time reduce runs, it sets accumulator to the empty 2-dimensional array. Within the method, the code checks whether the age property of the customer is above 18. If so, it pushes the customer to the first array. If the customer is underaged, they get pushed to the second array. In the end, the 2-dimensional array with the grouped customers is returned.

Reimplementing reduce

Time to reimplement reduce:

function reduce(callbackFn, initialValue) {
  let accumulator = initialValue ?? this[0];
  for (let i = 0; i < this.length; i++) {
    accumulator = callbackFn(accumulator, this[i], i, this);
  }
  return accumulator;
}

Array.prototype.reduce = reduce;

reduce iterates over all elements as all the other array methods do. In the beginning, the method needs to decide whether an initialValue was supplied. If not, the first element from the array is taken as such. After that, the accumulator is replaced with the result of calling the callback each time, and then returned in its final form in the end.

8. sort()

The name of sort already says it all. Whenever you want to sort an array, this is the method you need to call.

The following is the signature of the sort method:

sort(function (firstElement, secondElement) {
  // firstElement is the first element to compare
  // secondElement is the second element to compare
});

Example Use Case

Whenever you need to sort something, you found a use case for sort. Let’s, for example, try to sort your customers by age. For such a case, sort allows you to pass in a comparator function that you can use to tell sort how to properly order them:

const customers = getCustomers();

customers.sort((a, b) => customer.a - customer.b);

The callback function must return a number based on the order of the elements. It should return a value < 0 if a comes before b, 0 if both are equal, and a value > 0 if a comes after b.

Reimplementing sort

Reimplementing sort is a little difficult because, under the hood, runtimes are allowed to implement any sorting algorithm they seem fitting. There are only a few requirements, like that the algorithm must be stable. Usually, runtimes implement at least QuickSort, though, and sometimes change the implementation based on the elements within the array.

But let’s just reimplement Quick Sort in JavaScript to give you an idea:

function partition(array, left, right, compareFunction) {
  let pivot = array[Math.floor((right + left) / 2)];
  let i = left;
  let j = right;
    while (i <= j) {
        while (compareFunction(array[i], pivot) < 0) {
            i++;
        }
        while (compareFunction(array[j], pivot) > 0) {
            j--;
        }
        if (i <= j) {
            [array[i], array[j]] = [array[j], array[i]]
            i++;
            j--;
        }
    }
    return i;
}

function quickSort(array, left, right, compareFunction) {
  let index;
  if (array.length > 1) {
      index = partition(array, left, right, compareFunction);
      if (left < index - 1) {
          quickSort(array, left, index - 1, compareFunction);
      }
      if (index < right) {
          quickSort(array, index, right, compareFunction);
      }
  }
  return array;
}

function sort(compareFunction) {
  return quickSort(this, 0, this.length - 1, compareFunction);
}

Array.prototype.sort = sort;

This is only an exemplary implementation of sort, in this case, QuickSort. But it should give you a general idea.

9. find()

find is your search function. Whenever you look for something within an array, you can use find to retrieve the first element in the array that satisfies your conditions.

The following is the signature of the find method:

find(function (element, index, array) {
  // element is the current element
  // index is the current index
  // array is a reference to the array find works on
}, thisOverride);
// thisOverride is a way to override the semantical this within the callback function.
// If you set it to another object, calling this.anyThing would access anyThing within that
// object, and not the actual array.

Example Use Case

Imagine you try to find a customer with a specific name within all your customers. You could do it like this:

const customers = getCustomers();

const customerJohn = customers.find(
  (customer) => customer.forename === 'John'
);

In this case find returns the first user within the array that has the forename John. The important part is that find won’t return all customers with that name.

Reimplementing find

Time to reimplement find:

function find(callbackFn) {
  for (let i = 0; i < this.length; i++) {
    if (callbackFn(this[i], i, this)) {
      return this[i];
    }
  }
  return undefined;
}

Array.prototype.find = find;

find iterates over all elements as all the other array methods do. For every element, it checks whether the callback function returns true. If it does, it returns the element at that position. If it doesn’t return early, it returns undefined at the end.

10. findIndex()

findIndex is a method that you can use to get the index of an element within the array. Like find, it stops at the first element that satisfies the condition. It will thus only ever return the index of the first element satisfying the test.

The following is the signature of the findIndex method:

findIndex(function (element, index, array) {
  // element is the current element
  // index is the current index
  // array is a reference to the array find works on
}, thisOverride);
// thisOverride is a way to override the semantical this within the callback function.
// If you set it to another object, calling this.anyThing would access anyThing within that
// object, and not the actual array.

Example Use Case

Imagine you have all your customers sorted by age, and now you want to find the first customer with a forename of John. You could do it like this:

const customers = getCustomers();
const customersSortedByAge = sortByAge(customers);

const indexOfJohn customersSortedByAge.findIndex((customer) => customer.forename === 'John');
const customerJohn = customersSortedByAge[indexOfJohn];

In this case findIndex returns the index of the first user within the array that has the forename John. The important part is that findIndex won’t return all indices of customers with that name.

Reimplementing findIndex

Time to reimplement findIndex:

function findIndex(callbackFn) {
  for (let i = 0; i < this.length; i++) {
    if (callbackFn(this[i], i, this)) {
      return i;
    }
  }
  return -1;
}

Array.prototype.findIndex = findIndex;

findIndex iterates over all elements as all the other array methods do. You should notice the similarity to find. Instead of returning the element, only the index is returned when an element is found. Instead of undefined, if nothing is found, -1 is returned.

That’s It

You have now learned a little more about 10 of the more popular and important array methods in JavaScript. As you’ve learned, many of them are pretty similar, and they all have in common that they loop a lot. Other than that, they all have their similarities and differences, but this is what makes them so interesting to explore and learn.