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

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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

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Count odd and even numbers in an array using ↴

reduce() method → executes a reducer function, resulting in a single output value.

modulo operator % (remainder operator) → returns the remainder left over when one operand is divided by a second operand.

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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)

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Example 1 | Find the sum of the array ↴

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

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Example 2 | Find even numbers ↴

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.

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

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Modulo operator % (remainder operator) returns the remainder left over when one number is divided by a second number.

10 % 2 → remainder 0

10 % 3 → remainder 1

10 % 4 → remainder 2

10 % 5 → remainder 0

The modulo operator can be used to check whether a number is odd or even.

If a number is divisible by 2 (with no remainder) then it must be an even number.

If a number is not divisible by 2 (with a remainder) then it must be an odd number.

4 % 2 === 0; statement is true, remainder is zero, so 4 is an even number.

5 % 2 === 0; statement is false, remainder is not zero, so 5 is an odd number.

function checkNumber(num) {

if (num % 2 === 0) { if num divisible by 2

console.log(num + " is even"); if true

} else {

console.log(num + " is odd"); if false

}

}

checkNumber(2); → 2 is even

checkNumber(3); → 3 is odd

checkNumber(4); → 4 is even

checkNumber(5); → 5 is odd

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Initialize a variable to hold the array to count odd and even numbers.

const array1 = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 14, 16]; → user input

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Define a function countOddEven to count odd and even numbers in an array.

function countOddEven(arr) {}

The function takes an array of numbers as input arr and returns the count of odd and even numbers found.

Use the reduce() method to count odd and even numbers in an array.

return arr.reduce()

reduce() method iterates over the array of numbers and returns a single object with the accumulated counts of odd and even numbers.

callback function ↴

(acc, num) => {

if (num % 2 === 0) {

acc.evenCount++;

} else {

acc.oddCount++;

}

return acc;

}

callback function

(acc, num) => {}

Use the modulo operator to check if the current number num is even.

if (num % 2 === 0) {}

If true, increment even count by 1

acc.evenCount++

else if false, increment odd count by 1

acc.oddCount++

Return the accumulator for the next iteration.

return acc

initial value ↴

{ oddCount: 0, evenCount: 0 }

odd and even counts are initialized to 0

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() which will be the Object containing the counts of oddCount and evenCount

Call the function and store the results in the results variable.

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Call the function with ↴

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