Arrays are used to store multiple values in a single variable.
Each value is called an element, and each element has a numeric position in the array, known as its index.
Arrays are zero-indexed, meaning the first element is at index 0, the second at index 1, and so on.
Arrays can contain any data type, including numbers, strings, and objects.
const arr1 = [2, 4, 6]; array
arr1[0]; element at index 0 → 2
arr1[1]; element at index 1 → 4
arr1[2]; element at index 2 → 6
arr1[3]; element at index 3 → undefined index not found
Strings are a sequence of zero or more characters written inside quotes used to represent text.
Strings may consist of letters, numbers, symbols, words, or sentences.
Strings are immutable, they cannot be changed.
Each character in a string has an index.
The first character will be index 0 the second character will be index 1 and so on.
There are two ways to access an individual character in a string.
charAt() method
const str1 = "abc"; string
str1.charAt(0); character at index 0 → "a"
str1.charAt(1); character at index 1 → "b"
str1.charAt(2); character at index 2 → "c"
str1.charAt(3); character at index 3 → "" index not found
Alternatively use at() or slice() methods
bracket notation []
const str2 = "abc"; string
str2[0]; character at index 0 → "a"
str2[1]; character at index 1 → "b"
str2[2]; character at index 2 → "c"
str2[3]; character at index 3 → undefined index not found
Numbers are used to represent both integer and floating-point values.
Numbers are most commonly expressed in literal forms like 255 or 3.14159 ↴
let num1 = 5; → number
let num2 = 2.5; → number
let num3 = num1 + num2;
console.log(num3); returns ↴
7.5 → number
Merge two arrays with alternative values using ↴
reduce() method → executes a reducer function, resulting in a single output value.
push() method → adds specified elements to the end of an array and returns the new length of the array.
concat() method → merge two or more arrays.
slice() method → returns selected elements in an array, as a new array.
length property → set or return the number of elements in an array.
reduce() method iterates over each element in an array, and each iteration returns a single value, which is the accumulator.
When the iteration is finished, the accumulator value will be returned from the method.
It takes two parameters: a callback function and an optional initial value ↴
callback function first parameter.
initialValue second parameter. The accumulator is initialized to the first element of the array if no initial value is provided.
The callback function takes four parameters ↴
accumulator The value resulting from the previous call to callback function - required.
currentValue The value of the current element - required.
currentIndex Index position of currentValue in the array - optional.
Array The array reduce() was called upon - optional.
syntax
array.reduce(callback, initialValue); ↴
array.reduce((accumulator, currentValue, currentIndex, Array), initialValue)
Example 1 | Find the sum of the array ↴
const arr2 = [1, 2, 3, 4, 5, 6];
arr2.reduce((acc, cur) => acc + cur, 0); Initial value → 0
returns ↴
21
During each iteration, the current value cur will be added to the accumulator acc ↴
acc + cur
Iteration ↴
0 acc Initial value → 0
1 acc 0 → 0 + 1 = 1 → 1
2 acc 1 → 1 + 2 = 3 → 3
3 acc 3 → 3 + 3 = 6 → 6
4 acc 6 → 6 + 4 = 10 → 10
5 acc 10 → 10 + 5 = 15 → 15
6 acc 15 → 15 + 6 = 21 → 21
The return value becomes the value of the accumulator parameter acc on the next invocation of the callback function.
For the last invocation, the return value becomes the return value of reduce()
When the iteration is finished, the accumulator value will be returned ↴
21 → sum of the array
Example 2 | Find even numbers ↴
const arr3 = [11, 12, 13, 14, 15, 16, 17];
const evenNumbers = arr3.reduce((acc, cur) => {
return cur % 2 === 0 ? [...acc, cur] : acc;
}, []); Initial value → []
console.log(evenNumbers); returns ↴
[12, 14, 16]
During each iteration, the ternary operator evaluates the condition cur % 2 === 0
If current element cur is divisible by 2, it will be added to the accumulator array, acc
[...acc, cur] ↴
cur % 2 === 0 ? [...acc, cur] : acc
If current element cur not divisible by 2, the accumulator acc will be returned as is.
Iteration ↴
0 acc 11 → [] Initial value is an empty array
1 acc 11 → []
2 acc 12 → [12] 12 added to acc array
3 acc 13 → [12]
4 acc 14 → [12, 14] 14 added to acc array
5 acc 15 → [12, 14]
6 acc 16 → [12, 14, 16] 16 added to acc array
7 acc 17 → [12, 14, 16]
The return value becomes the value of the accumulator parameter acc on the next invocation of the callback function.
For the last invocation, the return value becomes the return value of reduce()
When the iteration is finished, the accumulator value will be returned ↴
[12, 14, 16] → even numbers
push() method adds new elements to the end of an array.
Add 4 to end of array.
const arr4 = [1, 2, 3];
arr4.push(4);
console.log(arr4); returns ↴
[1, 2, 3, 4] → 4 added to end of array
The push() method changes the length of the array.
arr4 is modified.
Using the spread operator creates a new array.
Add 4 to a new array.
const arr5 = [1, 2 , 3];
const arr6 = [...arr5, 4];
console.log(arr6); returns ↴
[1, 2, 3, 4] → 4 added to new array
console.log(arr5); returns ↴
[1, 2 ,3]
arr5 remains unchanged.
concat() method is used to merge two or more arrays.
This method does not change the existing arrays, but instead returns a new array.
const arr7 = [1, 2, 3, 4, 5];
const arr8 = [4, 5, 6, 7, 8];
const arr9 = arr7.concat(arr8);
console.log(arr9); returns ↴
[1, 2, 3, 4, 5, 4, 5, 6, 7, 8] → values are not unique
slice() method returns selected elements in an array, as a new array.
syntax ↴
slice(start) return a new array from start index to end of array
slice(start, end) return an array from start index to end index of array (exclusive).
Return a new array from index 1 to end of array.
const arr10 = [1, 2, 3, 4];
arr10.slice(1); start index is 1
returns ↴
[2, 3, 4]
Return a new array from index 1 to index 4 (exclusive).
const arr11 = [1, 2, 3, 4];
arr11.slice(1, 3); start index is 1 end index is 4 (not included)
returns ↴
[2, 3]
length property returns the number of elements in an array.
const arr12 = [1, 2, 3, 4, 5, 6];
arr12.length; returns ↴
6 → there are 6 elements in the array
Initialize variables to hold the two arrays to merge.
const array1 = [1, 3, 5, 7, 9]; → user input
const array2 = [2, 4, 6, 8, 10]; → user input
Define a function alternateMerge to merge two arrays into a new array with alternating values.
function alternateMerge(arr1, arr2)
The function takes two arrays as input arr1, arr2 and returns a new array containing the elements of the input arrays merged in alternating value. The original arrays are unchanged.
Use reduce() method to merge two arrays with alternating values.
return arr1.reduce()
reduce() method iterates over the first array and builds the merged result.
reduce(callbackFn, initialValue) ↴
return arr1.reduce((acc, curr, index) => (), [])
acc is the accumulator (initialized as an empty array)
curr is the current element
index is the current index
[] initialValue is an empty array
callback function ↴
(acc, curr, index) => {
acc.push(curr)
if (index < arr2.length) {
acc.push(arr2[index])
}
return acc
}
initial value ↴
[] empty array
Push the current element from the first array curr into the accumulator acc
acc.push(curr)
Check if there is a corresponding element in the second array.
if (index < arr2.length) {
Push the current element from the second array arr2[index] into the accumulator acc
acc.push(arr2[index])
Return the accumulator for the next iteration.
return acc
Initialize the accumulator as an empty array.
}, []
The return value becomes the value of the accumulator parameter acc on the next invocation of the callback function.
For the last invocation, the return value becomes the return value of reduce()
Concatenate any remaining elements from the second array.
Slice arr2 starting from the position derived from arr1.length to end of the array.
.concat(arr2.slice(arr1.length))
Merge two arrays with alternative values.
const array1 = [1, 3, 5, 7, 9];
const array2 = [2, 4, 6, 8, 10, 12, 14];
function alternateMerge(arr1, arr2) {
return arr1
.reduce((acc, curr, index) => {
acc.push(curr);
if (index < arr2.length) {
acc.push(arr2[index]);
}
return acc;
}, [])
.concat(arr2.slice(arr1.length));
}
call function
alternateMerge(array1, array2); returns ↴
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14]