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Array Methods2026-04-06

These methods are common in everyday JavaScript. They help you transform, search, and update arrays.

reduce​

reduce combines array items into a single final value.

Syntax:

let value = arr.reduce(function(accumulator, item, index, array) {
  // ...
}, [initial]);
  • accumulator stores the running result.
  • item is the current element.
  • index is the current index.
  • array is the original array.
  • initial is the starting accumulator value.

Example:

let arr = [1, 2, 3, 4, 5];

let result = arr.reduce((sum, current) => sum + current, 0);

alert(result); // 15

splice​

splice changes the original array. It can remove items, insert items, or do both.

Syntax:

arr.splice(start[, deleteCount, elem1, ..., elemN]);

Example 1: Replace elements​

let arr = ["I", "study", "JavaScript", "right", "now"];

// remove first 3 elements and replace them
arr.splice(0, 3, "Let's", "dance");

alert(arr); // ["Let's", "dance", "right", "now"]

Example 2: Remove elements​

let arr = ["I", "study", "JavaScript", "right", "now"];

// remove first 2 elements
let removed = arr.splice(0, 2);

alert(removed); // ["I", "study"]

slice​

slice returns a copy of part of an array and does not modify the original array.

let arr = ["t", "e", "s", "t"];

alert(arr.slice(1, 3)); // ["e", "s"]
alert(arr.slice(-2));   // ["s", "t"]

split and join​

Use split to convert a string into an array. Use join to convert an array back into a string.

let names = "Bilbo, Gandalf, Nazgul";

let arr = names.split(", ");
alert(arr); // ["Bilbo", "Gandalf", "Nazgul"]

let str = arr.join(" | ");
alert(str); // "Bilbo | Gandalf | Nazgul"

forEach​

forEach runs a function for each item. It is used for side effects (like logging or rendering), not for creating a new array.

Syntax:

arr.forEach(function(item, index, array) {
  // ... do something with an item
});

Example:

["Bilbo", "Gandalf", "Nazgul"].forEach((item, index, array) => {
  alert(`${item} is at index ${index} in ${array}`);
});

filter​

filter returns a new array with items that pass a condition.

Syntax:

let results = arr.filter(function(item, index, array) {
  // if true, item is pushed to results
  // returns empty array if nothing matches
});

Example:

let users = [
  { id: 1, name: "John" },
  { id: 2, name: "Pete" },
  { id: 3, name: "Mary" }
];

// returns first two users
let someUsers = users.filter(item => item.id < 3);

alert(someUsers.length); // 2

map​

map returns a new array where each item is transformed.

Syntax:

let result = arr.map(function(item, index, array) {
  // returns the new value
});

Example:

let lengths = ["Bilbo", "Gandalf", "Nazgul"].map(item => item.length);

alert(lengths); // [5, 7, 6]

sort​

sort reorders the array in place.

Without a compare function, values are sorted as strings. For numbers, pass a compare callback:

arr.sort((a, b) => a - b);
  • If result is < 0, a comes before b.
  • If result is > 0, a comes after b.
  • If result is 0, their order stays unchanged.

Example:

let numbers = [5, 2, 10, 1];
numbers.sort((a, b) => a - b);

alert(numbers); // [1, 2, 5, 10]

Quick Summary​

  • reduce: many values -> one value.
  • splice: add/remove/replace in the original array.
  • slice: copy part of an array.
  • split/join: convert between string and array.
  • forEach: run code for each item.
  • filter: keep matching items.
  • map: transform every item.
  • sort: reorder items (in place).

Important Note​

sort, reverse, and splice modify the original array.