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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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

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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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Remove duplicates from an array using ↴

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

includes() method → returns true if an array contains a specified value, otherwise returns false.

ternary operator → frequently used as an alternative to an if...else statement.

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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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includes() method determines whether an array includes a certain value among its entries, returning true or false

const arr4 = [1, 2, 3, 4, 5, 6];

arr4.includes(4); returns boolean ↴

true → 4 found in array

const arr5 = [1, 2, 3, 4, 5, 6];

arr5.includes(7); returns boolean ↴

false → 7 NOT found in array

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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

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spread syntax ... unpacks the elements of an iterable object, like arrays.

Add 4 to end of array using spread syntax.

const arr6 = [1, 2 ,3];

[...arr6, 4]; returns ↴

[1, 2, 3, 4] → 4 added to end of array

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Initialize an array from which to remove duplicates.

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Define a function removeDuplicates to return a new array with all duplicates removed.

function removeDuplicates(arr) {}

The function takes an array as input arr and returns a new array with all duplicate values removed.

Use reduce() method to iterate over the array and remove duplicates.

return arr.reduce()

reduce() method iterates through the array and executes a reducer function for each array element.

The reduce method executes a reducer function, resulting in a single output value, the acc array.

Using a ternary operator ↴

Check if curr is in the acc array.

acc.includes(curr)

If true, return acc unchanged.

If false, add current element curr to acc 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()

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Call the function with ↴

removeDuplicates(array1);

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Remove duplicates from array.

reduce() method iterates over the array and constructs a new array without duplicates.

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

Ternary operator ↴

acc.includes(curr) ? acc : [...acc, curr]

curr found in the in the array, acc remains unchanged ↴

acc unchanged

curr NOT found in acc add it to the array ↴

[...acc, curr]

returns ↴

[1, 2, 4, 3] No duplicate values. All values are unique.

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()

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Remove duplicates from array.