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
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
Find maximum number in an array using reduce method.
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
Ternary Operator
Ternary Operator is frequently used as an alternative to if...else statements.
if (condition) {
expressionIfTrue;
} else {
expressionIfFalse;
}
Ternary Operator takes three operands ↴
condition evaluates to true or false.
? question mark → expression to execute if the condition is truthy
: colon → expression to execute if the condition is falsy
condition ? expressionIfTrue : expressionIfFalse
let score = 75;
let result = score >= 55
? "You passed"
: "You failed";
console.log(result); returns ↴
You passed → first expression executed
Initialize an array from which to find the maximum number.
const array1 = [5, 4, 6, 8, 7]; → user input
Define a function findMaxNumber to find maximum number.
function findMaxNumber(arr) {}
The function takes an array as input arr and returns the number with the highest value found in that array.
Use reduce() method to iterate over the array and find the maximum number in the array.
return arr.reduce()
reduce() method iterates over the array of numbers and returns a single value, the maximum number found.
reduce(callbackFn, initialValue)
reduce((maxVal, curr) => (), arr[0])
maxVal is the accumulator
curr is the current element
arr[0] initialValue is first element of the array
callback function ↴
(maxVal, curr) => (curr > maxVal ? curr : maxVal)
initial value ↴
arr[0] first element of array
The callback function compares the current value curr with the maximum value found so far, maxVal
If curr is greater, it becomes the new maxVal
The return value becomes the value of the accumulator parameter maxVal on the next invocation of the callback function.
For the last invocation, the return value becomes the return value of reduce()
Call the function with ↴
findMaxNumber(array1);
Find the maximum number in an array.
const array1 = [5, 4, 6, 8, 7];
function findMaxNumber(arr) {
return arr.reduce((maxVal, curr) => (curr > maxVal ? curr : maxVal), arr[0]);
}
call function
findMaxNumber(array1); returns ↴
8
Alternative using built-in Math.max() method.
const arr = [5, 4, 6, 8, 7]; numbers only
Math.max(...arr); returns ↴
8