Splice, slice, pop and shift. Is the array sort method stable and not in-place? It's not easy to remember all JavaScript array methods and what the difference between them are. They are all synonyms, as if they where taken directly from a thesaurus.
This array cheat sheet lists all the array methods you usually need in JavaScript, and the ones you don't need, is not here. Simple as that! To make it even easier for you, I have based the cheat sheet on common use cases.
New challenge, try to list all array methods without forgetting one of them
Immutability
Before we begin, a notice about immutability. All functions used in this article are immutable, which shortly means, that you never mutate your original array when you use these methods. Immutability is very important in modern JavaScript, at least according to this Stack Overflow guy (and the rest of the Internet).
Even though each of these methods is immutable on their own, you are not totally safe against immutability. If you include mutable data types (arrays, objects, functions) in your immutable arrays, you can still mutate the mutable data within the array. We call such arrays to be shallowly immutable, in contrast to deeply immutable arrays which only includes immutable items.
Array Use Cases
- Add Element to Start of Array
- Add Element to End of Array
- Remove an Element From Start of Array
- Remove an Element From End of Array
- Insert Element at Index in Array
- Replace an Element at Index in Array
- Remove an Element at Index in Array
- Remove an Element By Value From an Array
- Remove Objects By Property From an Array
- Check if Array Includes an Element
- Check if Array Includes Object with Property
- Check if All Objects in an Array has a Property
- Convert Array to an Object
- Convert Array of Objects to an Object
- Convert Object to an Array
- Combine Two Arrays
- Sort an Array
- Sort an Array of Objects
- Reverse an Array
- Remove Duplicates From an Array
1. Add Element to Start of Array
To prepend an item to the beginning of an array, use array spreading.
const arr = [1, 2, 3];
const result = [0, ...arr];
console.log(result);
// [0, 1, 2, 3]
Do not use the unshift method, that will mutate the original array.
2. Add Element to End of Array
To append an item to the end of an array, use array spreading.
const arr = [1, 2, 3];
const result = [...arr, 4];
console.log(result);
// [1, 2, 3, 4]
Do not use the push method, that will mutate the original array.
3. Remove an Element From Start of Array
To remove the first item in an array, use slice method.
const arr = [1, 2, 3];
// Keep index 1 and everything after that.
const result = arr.slice(1);
console.log(result);
// [2, 3]
Do not use the shift or splice methods, they will mutate the original array.
4. Remove an Element From End of Array
To remove the last item in an array, use slice method.
const arr = [1, 2, 3];
// Keep index 0 and everything after that, except one element at the array.
const result = arr.slice(0, -1);
console.log(result);
// [1, 2]
Do not use the pop or splice methods, they will mutate the original array.
5. Insert Element at Index in Array
To add an item at a specific index of an array, use the toSpliced method.
const arr = [1, 2, 3];
// Keep index 1, delete 0 elements, add the element "one point five" and keep everything after that.
const result = arr.toSpliced(1, 0, "one point five");
console.log(result);
// [1, "one point five", 2, 3]
Do not use the splice method, that will mutate the original array.
6. Replace an Element at Index in Array
To replace an item from some index in an array, use the toSpliced or with method.
const arr = [1, 2, 3];
// Using toSpliced.
// Keep index 1, delete 1 elements, add the element "two" and keep everything after that.
const result1 = arr.toSpliced(1, 1, "two");
console.log(result1);
// [1, "two", 3]
// Using with.
// Copy the old array arr with index 1 replaced with "two".
const result2 = arr.with(1, "two");
console.log(result2);
// [1, "two", 3]
Do not use the splice method, that will mutate the original array.
7. Remove an Element at Index in Array
To remove an item from an array, use the toSpliced method.
const arr = [1, 2, 3];
// At index 1, delete 1 elements.
const result = arr.toSpliced(1, 1);
console.log(result);
// [1, 3]
Do not use the splice method, that will mutate the original array.
8. Remove an Element By Value From an Array
To remove a specific value from an array, use the filter method.
const arr = [1, 2, 3];
const result = arr.filter((element) => element !== 2);
console.log(result);
// [1, 3]
Do not use the indexOf together with splice method, that will mutate the original array.
9. Remove Objects By Property From an Array
To remove an object with a specific attribute from an array, use the filter method.
const arr = [{ num: 1 }, { num: 2 }, { num: 3 }];
const result = arr.filter((obj) => obj.num !== 2);
console.log(result);
// [{ num: 1 }, { num: 3 }]
Do not use the findIndex together with splice method, that will mutate the original array.
10. Check if Array Includes an Element
To check if an array contains a value, use includes method.
const arr = [1, 2, 3];
const result = arr.includes(2);
console.log(result);
// true
11. Check if Array Includes Object with Property
To check if an array contains an object with a property, use some method.
const arr = [{ num: 1 }, { num: 2 }, { num: 3 }];
const result = arr.some((obj) => obj.num === 2);
console.log(result);
// true
12. Check if All Objects in an Array has a Property
To check if every object in an array has a property, use every method.
const arr1 = [{ num: 1 }, { num: 2 }, { num: 3 }];
const result1 = arr1.every((obj) => obj.num === 2);
console.log(result1);
// false
const arr2 = [{ num: 2 }, { num: 2 }, { num: 2 }];
const result2 = arr2.every((obj) => obj.num === 2);
console.log(result2);
// true
13. Convert Array to an Object
To convert an array to a custom object, use reduce method.
// A function which maps a key to a value.
const arr1 = [1, 2, 3];
const result1 = arr1.reduce((acc, cur, index) => {
acc[`attr${index}`] = cur;
return acc;
}, {});
console.log(result1);
// { attr0: 1, attr1: 2, attr2: 3 }
// A function which count occurrences could look like this.
const arr2 = ["a", "b", "c", "c"];
const result2 = arr2.reduce((acc, cur) => {
if (acc[cur]) {
acc[cur] += 1;
} else {
acc[cur] = 1;
}
return acc;
}, {});
console.log(result2);
// { a: 1, b: 1, c: 2 })
// A function which maps elements in array to boolean values can look like this.
// I.e. convert array to object keys.
const arr3 = ["a", "b", "c"];
const truthValues = ["b", "c"];
const result3 = arr3.reduce((acc, cur) => {
acc[cur] = truthValues.includes(cur);
return acc;
}, {});
console.log(result3);
// { a: false, b: true, c: true })
14. Convert Array of Objects to an Object
To convert an array of objects to an object, use Object.assign method and array spread syntax.
const arr = [{ attr1: 1 }, { attr2: 2 }, { attr3: 3 }];
const result = Object.assign({}, ...arr);
console.log(result);
// { attr1: 1, attr2: 2, attr3: 3 }
15. Convert Object to an Array
To create an array from an object, use Object.keys, Object.values, or Object.entries, potentially together with a map method.
const obj = { a: 1, b: 2, c: 3 };
// Array of keys.
const result1 = Object.keys(obj);
console.log(result1);
// ["a", "b", "c"]
// Array of values.
const result2 = Object.values(obj);
console.log(result2);
// [1, 2, 3]
// Array of key-value objects.
const result3 = Object.entries(obj).map(([key, value]) => ({ key, value }));
console.log(result3);
// [{ key: "a", value: 1 }, { key: "b", value: 2 }, { key: "c", value: 3 }]
In some cases, it is useful to chain some map and filter methods to modify and filter out values.
const obj = { a: 1, b: 2, c: 3 };
// Array of squared values greater than 3.
const result1 = Object.values(obj)
.map((value) => value * value)
.filter((value) => value > 3);
console.log(result1);
// [4, 9]
// Array of key-value objects which has a value greater than 1.
const result2 = Object.entries(obj)
.map(([key, value]) => ({ key, value }))
.filter((keyValueObj) => keyValueObj.value > 1);
console.log(result2);
// [{ key: "b", value: 2 }, { key: "c", value: 3 }]
16. Combine Two Arrays
To combine two JavaScript arrays, use concat method or the spread syntax for arrays.
const arr1 = [1, 2, 3];
const arr2 = [4, 5, 6];
// Concat method is faster.
const combinedArray1 = arr1.concat(arr2);
console.log(combinedArray1);
// [1, 2, 3, 4, 5, 6]
// Spread syntax may be more readable.
const combinedArray2 = [...arr1, ...arr2];
console.log(combinedArray2);
// [1, 2, 3, 4, 5, 6]
Do not use the push method, that will mutate the original array.
17. Sort an Array
If you want to sort an array by value, use toStorted method.
The toStorted method is stable, meaning that it keeps the order of elements which are equal to each other intact. The method is not in-place, which usually is a good thing, since it means that it won't mutate the existing array.
// To sort strings.
let arr1 = ["b", "c", "a"];
const result1 = arr1.toSorted();
console.log(result1);
// ["a", "b", "c"]
// Note: Numbers are sorted by their toString value,
// not by their numerical value!
const arr2 = [10, 1, 5];
const result2 = arr2.toSorted();
console.log(result2);
// [1, 10, 5]
// To sort numbers, use a comparator.
const arr3 = [10, 1, 5];
const result3 = arr3.toSorted((a, b) => a - b);
console.log(result3);
// [1, 5, 10]
Do not use the sort method, that will mutate the original array since it does in-place sorting.
18. Sort an Array of Objects
To sort an array by value, use toStorted method with a comparator. A comparator is a function which determines which of two values that should be sorted first.
The toStorted method is stable, meaning that it keeps the order of elements which are equal to each other intact. The method is not in-place, which usually is a good thing, since it means that it won't mutate the existing array.
const arr = [{ num: 3 }, { num: 1 }, { num: 2 }];
// ObjA will be sorted before objB if comparator returns a positive value.
const byNumberAttribute = (objA, objB) => objA.num - objB.num;
const result1 = arr.toSorted(byNumberAttribute);
console.log(result1);
// [{ num: 1 }, { num: 2 }, { num: 3 }]
// More generic comparator.
const byAttribute = (attr) => (objA, objB) => objA[attr] - objB[attr];
const result2 = arr.toSorted(byAttribute("num"));
console.log(result2);
// [{ num: 1 }, { num: 2 }, { num: 3 }]
// Note. The comparator function must return an integer value.
// If you need to sort other data types, return 1, 0 or -1.
const arr3 = [{ letter: "c" }, { letter: "a" }, { letter: "b" }];
const alphabetically = (objA, objB) => {
if (objA.letter < objB.letter) {
return -1;
}
if (objA.letter > objB.letter) {
return 1;
}
// objA === objB
return 0;
};
const result3 = arr3.toSorted(alphabetically);
console.log(result3);
// [{ letter: 'a' }, { letter: 'b' }, { letter: 'c' }]
Do not use the sort method, that will mutate the original array.
19. Reverse an Array
To reverse all values in an array, use toReversed method.
const arr = [1, 2, 3];
const result = arr.toReversed(2);
console.log(result);
// [3, 2, 1]
20. Remove Duplicates From an Array
To remove duplicated elements in an array, use filter method or a set.
const arr = [1, 2, 3, 2, 1];
// Using filter method.
const result1 = arr.filter((item, index) => arr.indexOf(item) === index);
console.log(result1);
// [1, 2, 3]
// Using a set.
const result2 = [...new Set(arr)];
console.log(result2);
// [1, 2, 3]
Top comments (12)
Thanks for reading! Let me know if I should post more articles like this by writing a comment or reacting with some of the reactions ❤️🦄 🔥
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Nice and complete collection of array methods I will gladly refer to in the future. Potentially also saves you alot of time for these ivory tower code interviews.
Wonderful
Very interesting examples
Excellent article, thank you.
Great article. Thanks!
very helpful article, thank you.
Great post!
Very helpfull article, thank you!
Very nice post. I have one minor point on the term "in-place" in items 17 and 18 of your post. The specification doesn't actually indicate whether or not either of sort or toSorted uses an in-place sorting algorithm. That decision appears left to the implementation so probably varies by browser or JS runtime. Note that Mozilla claims sort uses an in-place sorting algorithm but that isn't actually in the specification. Returning a new sorted copy of the array, like done by toSorted, instead of modifying original array isn't the same thing as being not in-place. Likewise sorting the original array itself, like done by sort, doesn't require an in-place sorting algorithm. You can do either of these cases with an in-place algorithm or one that is not in-place. It actually looks like the specification indicates that both of these use the same sorting algorithm, without specifying what that algorithm is. The only specific requirement I see is that it must be stable. So these are either both in-place or neither are in-place.
An in-place sorting algorithm is one that uses extra storage that is at most logarithmic in the array length (e.g. insertion sort, heap sort, mergesort with an in-place merge, introsort, etc). If it uses more auxiliary memory than that then it is not in-place, such as mergesort (most fast implementations use linear extra memory during the merge).
In comments on some SO threads, there are arguments over what sorting algorithm is used, or whether some SO answers are out-dated, etc mainly because browsers are free to implement whatever sorting algorithm they wish so the answer varies. It seems that Timsort and Introsort are both currently used in some JS implementations, both of these are in-place sorting algorithms. It is just toSorted sorts and returns a copy rather than sorting the original.
Edit: I can't actually confirm whether Introsort has been used. Saw some claims of that on SO, but is a bit doubtful since it isn't stable and specification requires a stable sort. It looks like Chrome switched from Quicksort to TimSort. Quicksort isn't stable so interesting they had been using it previously.
Thanks for a great comment, you are right. And it's nice to point out the data structures and algorithm perspective of it as well.
I didn't really mention anything about memory consumption since I felt the important thing in this article was about modifying the array or not, which a normal sort for sure does.
I actually checked that Mozilla page when writing the article, but as you mention, they only refer to the ECMA specification with the stability, not for their in-place statement. So that can only speak for Mozilla implementation.
I'm surprised that until they switched to Timsort that Chrome was using a non-stable sort when the specification indicates stable. In-place vs not in-place doesn't matter much unless the array is large enough to stress any memory constraints. Whereas stable vs non-stable may lead to bugs in the application if it relies on stability.