FindIndex(), FindLast(), FindLastIndex(),indexOf(),lastIndexOf() in JavaScript.

FindIndex():

They return the first element that matches a condition. So how like the “find” method but they return the index of element. And it didn’t modify it original array.

Direction: left → right

const find = [5, 12, 8, 130, 44];
const val = find.findIndex((i) => i > 10);
console.log(find);   // [ 5, 12, 8, 130, 44 ]
console.log(val);   // 1 

If they didn’t match they return “-1”.

FindLast():

Return the last value of last element that satisfy or matches the condition. And also they didn’t modify it original array.

Direction: right → left

const find = [5, 12, 8, 130, 44];

const val = find.findLast((i) => i > 40);
console.log(find);    // [ 5, 12, 8, 130, 44 ]

console.log(val);    // 44

This one is newer (ES2023). Works in modern browsers / Node 18+

FindLastIndex():

Returns the index of the last element that matches a condition. And this also didn’t modify his original array.

Direction: right → left

const find = [1, 3, 7, 5, 3, 7];
const val = find.findLastIndex((i) => i > 3);  // they gave us the index of last element that match the condition

console.log(find);   // [ 1, 3, 7, 5, 3, 7 ]
console.log(val);    // 5

Another example of findLastIndex()

const find = ["apple", "mango", "banana", "orange", "apple", "grapes"];
const val = find.findLastIndex((i) => i === "apple");

console.log(find);   // ["apple", "mango", "banana", "orange", "apple", "grapes"];
console.log(val);  // 4

In last i know you confuse about direction: You understand it from “left to right” but you confuse in “right to left” direction.

const num = [1, 2, 3, 4, 5, 6, 7];
      index= 0, 1, 2, 3, 4, 5, 6;     // Array indexes are fixed and never change.

// but findLastIndex() start from right to left , but index are fixed and they never change.

indexOf():

They gave us the index of first element that satisfy the condition. They didnot modify it original array.

Direction: left → right

const find = [1, 3, 7, 5, 3, 7];

const val = find.indexOf(3);
console.log(find);   // [1, 3, 7, 5, 3, 7];
console.log(val);  // 1

lastIndexOf: They gave us the index last element that satisfy the condition. They also didn’t modify his original array.

Direction: right → left

const find = [1, 3, 7, 5, 3, 7];

const val = find.lastIndexOf(3);
console.log(find);   // [1, 3, 7, 5, 3, 7];
console.log(val);   //  4 

Thank you so much for your time and I hope you learn something new from this article.