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Array methods

Arrays provide a lot of methods. To make things easier, in this chapter they are split into groups.

Add/remove items

We already know methods that add and remove items from the beginning or the end:

  • arr.push(...items) – adds items to the end,
  • arr.pop() – extracts an item from the end,
  • arr.shift() – extracts an item from the beginning,
  • arr.unshift(...items) – adds items to the beginning.

Here are a few others.


How to delete an element from the array?

The arrays are objects, so we can try to use delete:

The element was removed, but the array still has 3 elements, we can see that arr.length == 3.

That’s natural, because delete obj.key removes a value by the key. It’s all it does. Fine for objects. But for arrays we usually want the rest of elements to shift and occupy the freed place. We expect to have a shorter array now.

So, special methods should be used.

The arr.splice method is a swiss army knife for arrays. It can do everything: insert, remove and replace elements.

The syntax is:

It modifies arr starting from the index start: removes deleteCount elements and then inserts elem1, ..., elemN at their place. Returns the array of removed elements.

This method is easy to grasp by examples.

Let’s start with the deletion:

Easy, right? Starting from the index 1 it removed 1 element.

In the next example we remove 3 elements and replace them with the other two:

Here we can see that splice returns the array of removed elements:

The splice method is also able to insert the elements without any removals. For that we need to set deleteCount to 0:

Negative indexes allowed

Here and in other array methods, negative indexes are allowed. They specify the position from the end of the array, like here:


The method arr.slice is much simpler than similar-looking arr.splice.

The syntax is:

It returns a new array copying to it all items from index start to end (not including end). Both start and end can be negative, in that case position from array end is assumed.

It’s similar to a string method str.slice, but instead of substrings it makes subarrays.

For instance:

We can also call it without arguments: arr.slice() creates a copy of arr. That’s often used to obtain a copy for further transformations that should not affect the original array.


The method arr.concat creates a new array that includes values from other arrays and additional items.

The syntax is:

It accepts any number of arguments – either arrays or values.

The result is a new array containing items from arr, then arg1arg2 etc.

If an argument argN is an array, then all its elements are copied. Otherwise, the argument itself is copied.

For instance:

Normally, it only copies elements from arrays. Other objects, even if they look like arrays, are added as a whole:

…But if an array-like object has a special Symbol.isConcatSpreadable property, then it’s treated as an array by concat: its elements are added instead:

Iterate: forEach

The arr.forEach method allows to run a function for every element of the array.

The syntax:

For instance, this shows each element of the array:

And this code is more elaborate about their positions in the target array:

The result of the function (if it returns any) is thrown away and ignored.

Searching in array

Now let’s cover methods that search in an array.

indexOf/lastIndexOf and includes

The methods arr.indexOf and arr.includes have the similar syntax and do essentially the same as their string counterparts, but operate on items instead of characters:

  • arr.indexOf(item, from) – looks for item starting from index from, and returns the index where it was found, otherwise -1.
  • arr.includes(item, from) – looks for item starting from index from, returns true if found.

Usually these methods are used with only one argument: the item to search. By default, the search is from the beginning.

For instance:

Please note that indexOf uses the strict equality === for comparison. So, if we look for false, it finds exactly false and not the zero.

If we want to check if item exists in the array, and don’t need the index, then arr.includes is preferred.

The method arr.lastIndexOf is the same as indexOf, but looks for from right to left.

The includes method handles NaN correctly

A minor, but noteworthy feature of includes is that it correctly handles NaN, unlike indexOf:

That’s because includes was added to JavaScript much later and uses the more up to date comparison algorithm internally.

find and findIndex/findLastIndex

Imagine we have an array of objects. How do we find an object with the specific condition?

Here the arr.find(fn) method comes in handy.

The syntax is:

The function is called for elements of the array, one after another:

  • item is the element.
  • index is its index.
  • array is the array itself.

If it returns true, the search is stopped, the item is returned. If nothing found, undefined is returned.

For example, we have an array of users, each with the fields id and name. Let’s find the one with id == 1:

In real life arrays of objects is a common thing, so the find method is very useful.

Note that in the example we provide to find the function item => item.id == 1 with one argument. That’s typical, other arguments of this function are rarely used.

The arr.findIndex method has the same syntax, but returns the index where the element was found instead of the element itself. The value of -1 is returned if nothing is found.

The arr.findLastIndex method is like findIndex, but searches from right to left, similar to lastIndexOf.

Here’s an example:


The find method looks for a single (first) element that makes the function return true.

If there may be many, we can use arr.filter(fn).

The syntax is similar to find, but filter returns an array of all matching elements:

For instance:

Transform an array

Let’s move on to methods that transform and reorder an array.


The arr.map method is one of the most useful and often used.

It calls the function for each element of the array and returns the array of results.

The syntax is:

For instance, here we transform each element into its length:


The call to arr.sort() sorts the array in place, changing its element order.

It also returns the sorted array, but the returned value is usually ignored, as arr itself is modified.

For instance:

Did you notice anything strange in the outcome?

The order became 1, 15, 2. Incorrect. But why?

The items are sorted as strings by default.

Literally, all elements are converted to strings for comparisons. For strings, lexicographic ordering is applied and indeed "2" > "15".

To use our own sorting order, we need to supply a function as the argument of arr.sort().

The function should compare two arbitrary values and return:

For instance, to sort as numbers:

Now it works as intended.

Let’s step aside and think what’s happening. The arr can be array of anything, right? It may contain numbers or strings or objects or whatever. We have a set of some items. To sort it, we need an ordering function that knows how to compare its elements. The default is a string order.

The arr.sort(fn) method implements a generic sorting algorithm. We don’t need to care how it internally works (an optimized quicksort or Timsort most of the time). It will walk the array, compare its elements using the provided function and reorder them, all we need is to provide the fn which does the comparison.

By the way, if we ever want to know which elements are compared – nothing prevents from alerting them:

The algorithm may compare an element with multiple others in the process, but it tries to make as few comparisons as possible.

A comparison function may return any number

Actually, a comparison function is only required to return a positive number to say “greater” and a negative number to say “less”.

That allows to write shorter functions:

Arrow functions for the best

Remember arrow functions? We can use them here for neater sorting:

This works exactly the same as the longer version above.

Use localeCompare for strings

Remember strings comparison algorithm? It compares letters by their codes by default.

For many alphabets, it’s better to use str.localeCompare method to correctly sort letters, such as Ö.

For example, let’s sort a few countries in German:


The method arr.reverse reverses the order of elements in arr.

For instance:

It also returns the array arr after the reversal.

split and join

Here’s the situation from real life. We are writing a messaging app, and the person enters the comma-delimited list of receivers: John, Pete, Mary. But for us an array of names would be much more comfortable than a single string. How to get it?

The str.split(delim) method does exactly that. It splits the string into an array by the given delimiter delim.

In the example below, we split by a comma followed by space:

The split method has an optional second numeric argument – a limit on the array length. If it is provided, then the extra elements are ignored. In practice it is rarely used though

Split into letters

The call to split(s) with an empty s would split the string into an array of letters:

The call arr.join(glue) does the reverse to split. It creates a string of arr items joined by glue between them.

For instance:


When we need to iterate over an array – we can use forEachfor or for..of.

When we need to iterate and return the data for each element – we can use map.

The methods arr.reduce and arr.reduceRight also belong to that breed, but are a little bit more intricate. They are used to calculate a single value based on the array.

The syntax is:

The function is applied to all array elements one after another and “carries on” its result to the next call.


  • accumulator – is the result of the previous function call, equals initial the first time (if initial is provided).
  • item – is the current array item.
  • index – is its position.
  • array – is the array.

As function is applied, the result of the previous function call is passed to the next one as the first argument.

So, the first argument is essentially the accumulator that stores the combined result of all previous executions. And at the end it becomes the result of reduce.

Sounds complicated?

The easiest way to grasp that is by example.

Here we get a sum of an array in one line:

The function passed to reduce uses only 2 arguments, that’s typically enough.

Let’s see the details of what’s going on.

  1. On the first run, sum is the initial value (the last argument of reduce), equals 0, and current is the first array element, equals 1. So the function result is 1.
  2. On the second run, sum = 1, we add the second array element (2) to it and return.
  3. On the 3rd run, sum = 3 and we add one more element to it, and so on…

The calculation flow:

Or in the form of a table, where each row represents a function call on the next array element:

the first call011
the second call123
the third call336
the fourth call6410
the fifth call10515

Here we can clearly see how the result of the previous call becomes the first argument of the next one.

We also can omit the initial value:

The result is the same. That’s because if there’s no initial, then reduce takes the first element of the array as the initial value and starts the iteration from the 2nd element.

The calculation table is the same as above, minus the first row.

But such use requires an extreme care. If the array is empty, then reduce call without initial value gives an error.

Here’s an example:

So it’s advised to always specify the initial value.

The method arr.reduceRight does the same, but goes from right to left.


Arrays do not form a separate language type. They are based on objects.

So typeof does not help to distinguish a plain object from an array:

…But arrays are used so often that there’s a special method for that: Array.isArray(value). It returns true if the value is an array, and false otherwise.

Most methods support “thisArg”

Almost all array methods that call functions – like findfiltermap, with a notable exception of sort, accept an optional additional parameter thisArg.

That parameter is not explained in the sections above, because it’s rarely used. But for completeness we have to cover it.

Here’s the full syntax of these methods:

The value of thisArg parameter becomes this for func.

For example, here we use a method of army object as a filter, and thisArg passes the context:

If in the example above we used users.filter(army.canJoin), then army.canJoin would be called as a standalone function, with this=undefined, thus leading to an instant error.

A call to users.filter(army.canJoin, army) can be replaced with users.filter(user => army.canJoin(user)), that does the same. The latter is used more often, as it’s a bit easier to understand for most people.


A cheat sheet of array methods:

  • To add/remove elements:
    • push(...items) – adds items to the end,
    • pop() – extracts an item from the end,
    • shift() – extracts an item from the beginning,
    • unshift(...items) – adds items to the beginning.
    • splice(pos, deleteCount, ...items) – at index pos deletes deleteCount elements and inserts items.
    • slice(start, end) – creates a new array, copies elements from index start till end (not inclusive) into it.
    • concat(...items) – returns a new array: copies all members of the current one and adds items to it. If any of items is an array, then its elements are taken.
  • To search among elements:
    • indexOf/lastIndexOf(item, pos) – look for item starting from position pos, return the index or -1 if not found.
    • includes(value) – returns true if the array has value, otherwise false.
    • find/filter(func) – filter elements through the function, return first/all values that make it return true.
    • findIndex is like find, but returns the index instead of a value.
  • To iterate over elements:
    • forEach(func) – calls func for every element, does not return anything.
  • To transform the array:
    • map(func) – creates a new array from results of calling func for every element.
    • sort(func) – sorts the array in-place, then returns it.
    • reverse() – reverses the array in-place, then returns it.
    • split/join – convert a string to array and back.
    • reduce/reduceRight(func, initial) – calculate a single value over the array by calling func for each element and passing an intermediate result between the calls.
  • Additionally:
    • Array.isArray(value) checks value for being an array, if so returns true, otherwise false.

Please note that methods sortreverse and splice modify the array itself.

These methods are the most used ones, they cover 99% of use cases. But there are few others:

  • arr.some(fn)/arr.every(fn) check the array.The function fn is called on each element of the array similar to map. If any/all results are true, returns true, otherwise false.These methods behave sort of like || and && operators: if fn returns a truthy value, arr.some() immediately returns true and stops iterating over the rest of items; if fn returns a falsy value, arr.every() immediately returns false and stops iterating over the rest of items as well.We can use every to compare arrays:function arraysEqual(arr1, arr2) { return arr1.length === arr2.length && arr1.every((value, index) => value === arr2[index]); } alert( arraysEqual([1, 2], [1, 2])); // true
  • arr.fill(value, start, end) – fills the array with repeating value from index start to end.
  • arr.copyWithin(target, start, end) – copies its elements from position start till position end into itself, at position target (overwrites existing).
  • arr.flat(depth)/arr.flatMap(fn) create a new flat array from a multidimensional array.

For the full list, see the manual.

From the first sight it may seem that there are so many methods, quite difficult to remember. But actually that’s much easier.

Look through the cheat sheet just to be aware of them. Then solve the tasks of this chapter to practice, so that you have experience with array methods.

Afterwards whenever you need to do something with an array, and you don’t know how – come here, look at the cheat sheet and find the right method. Examples will help you to write it correctly. Soon you’ll automatically remember the methods, without specific efforts from your side.


Translate border-left-width to borderLeftWidth

importance: 5

Write the function camelize(str) that changes dash-separated words like “my-short-string” into camel-cased “myShortString”.

That is: removes all dashes, each word after dash becomes uppercased.


P.S. Hint: use split to split the string into an array, transform it and join back.

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Filter range

importance: 4

Write a function filterRange(arr, a, b) that gets an array arr, looks for elements with values higher or equal to a and lower or equal to b and return a result as an array.

The function should not modify the array. It should return the new array.

For instance:

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Filter range “in place”

importance: 4

Write a function filterRangeInPlace(arr, a, b) that gets an array arr and removes from it all values except those that are between a and b. The test is: a ≤ arr[i] ≤ b.

The function should only modify the array. It should not return anything.

For instance:

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Sort in decreasing order

importance: 4


Copy and sort array

importance: 5

We have an array of strings arr. We’d like to have a sorted copy of it, but keep arr unmodified.

Create a function copySorted(arr) that returns such a copy.


We can use slice() to make a copy and run the sort on it:

Create an extendable calculator

importance: 5

Create a constructor function Calculator that creates “extendable” calculator objects.

The task consists of two parts.

  1. First, implement the method calculate(str) that takes a string like "1 + 2" in the format “NUMBER operator NUMBER” (space-delimited) and returns the result. Should understand plus + and minus -.Usage example:let calc = new Calculator; alert( calc.calculate("3 + 7") ); // 10
  2. Then add the method addMethod(name, func) that teaches the calculator a new operation. It takes the operator name and the two-argument function func(a,b) that implements it.For instance, let’s add the multiplication *, division / and power **:let powerCalc = new Calculator; powerCalc.addMethod("*", (a, b) => a * b); powerCalc.addMethod("/", (a, b) => a / b); powerCalc.addMethod("**", (a, b) => a ** b); let result = powerCalc.calculate("2 ** 3"); alert( result ); // 8
  • No parentheses or complex expressions in this task.
  • The numbers and the operator are delimited with exactly one space.
  • There may be error handling if you’d like to add it.

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  • Please note how methods are stored. They are simply added to this.methods property.
  • All tests and numeric conversions are done in the calculate method. In future it may be extended to support more complex expressions.

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Map to names

importance: 5

You have an array of user objects, each one has user.name. Write the code that converts it into an array of names.

For instance:


Map to objects

importance: 5

You have an array of user objects, each one has namesurname and id.

Write the code to create another array from it, of objects with id and fullName, where fullName is generated from name and surname.

For instance:

So, actually you need to map one array of objects to another. Try using => here. There’s a small catch.solution

Please note that in the arrow functions we need to use additional brackets.

We can’t write like this:

As we remember, there are two arrow functions: without body value => expr and with body value => {...}.

Here JavaScript would treat { as the start of function body, not the start of the object. The workaround is to wrap them in the “normal” brackets:

Now fine.

Sort users by age

importance: 5

Write the function sortByAge(users) that gets an array of objects with the age property and sorts them by age.

For instance:


Shuffle an array

importance: 3

Write the function shuffle(array) that shuffles (randomly reorders) elements of the array.

Multiple runs of shuffle may lead to different orders of elements. For instance:

All element orders should have an equal probability. For instance, [1,2,3] can be reordered as [1,2,3] or [1,3,2] or [3,1,2] etc, with equal probability of each case.solution

The simple solution could be:

That somewhat works, because Math.random() - 0.5 is a random number that may be positive or negative, so the sorting function reorders elements randomly.

But because the sorting function is not meant to be used this way, not all permutations have the same probability.

For instance, consider the code below. It runs shuffle 1000000 times and counts appearances of all possible results:

An example result (depends on JS engine):

We can see the bias clearly: 123 and 213 appear much more often than others.

The result of the code may vary between JavaScript engines, but we can already see that the approach is unreliable.

Why it doesn’t work? Generally speaking, sort is a “black box”: we throw an array and a comparison function into it and expect the array to be sorted. But due to the utter randomness of the comparison the black box goes mad, and how exactly it goes mad depends on the concrete implementation that differs between engines.

There are other good ways to do the task. For instance, there’s a great algorithm called Fisher-Yates shuffle. The idea is to walk the array in the reverse order and swap each element with a random one before it:

Let’s test it the same way:

The example output:

Looks good now: all permutations appear with the same probability.

Also, performance-wise the Fisher-Yates algorithm is much better, there’s no “sorting” overhead.

Get average age

importance: 4

Write the function getAverageAge(users) that gets an array of objects with property age and returns the average age.

The formula for the average is (age1 + age2 + ... + ageN) / N.

For instance:


Filter unique array members

importance: 4

Let arr be an array.

Create a function unique(arr) that should return an array with unique items of arr.

For instance:

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Let’s walk the array items:

  • For each item we’ll check if the resulting array already has that item.
  • If it is so, then ignore, otherwise add to results.

The code works, but there’s a potential performance problem in it.

The method result.includes(str) internally walks the array result and compares each element against str to find the match.

So if there are 100 elements in result and no one matches str, then it will walk the whole result and do exactly 100 comparisons. And if result is large, like 10000, then there would be 10000 comparisons.

That’s not a problem by itself, because JavaScript engines are very fast, so walk 10000 array is a matter of microseconds.

But we do such test for each element of arr, in the for loop.

So if arr.length is 10000 we’ll have something like 10000*10000 = 100 millions of comparisons. That’s a lot.

So the solution is only good for small arrays.

Further in the chapter Map and Set we’ll see how to optimize it.

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Create keyed object from array

importance: 4

Let’s say we received an array of users in the form {id:..., name:..., age:... }.

Create a function groupById(arr) that creates an object from it, with id as the key, and array items as values.

For example:

Such function is really handy when working with server data.

In this task we assume that id is unique. There may be no two array items with the same id.

Please use array .reduce method in the solution.

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