How to write JavaScript indexOf and lastIndexOf array methods from scratch

indexOf

Returns first index of the found element otherwise -1.

Array.prototype._indexOf = function(searchElement, fromIndex = 0) {
    for(let i = fromIndex; i < this.length; i++) {
        if(searchElement === this[i]) {
            return i
        }
    }
    return -1
}

const fruits = [ 'apple', 'mango', 'banana', 'mango', 'orange' ]

console.log(fruits._indexOf('mango'))

// Output
// 1

• function accepts two parameters searchElement and optional fromIndex

• if found return index otherwise return -1

lastIndexOf

Returns the lastIndex if elements are found otherwise returns -1.

Array.prototype._lastIndexOf = function(searchElement, fromIndex = 0) {
    let index = -1;
    for(let i = fromIndex; i < this.length; i++) {
        if(searchElement === this[i]) {
            index = i
        }
    }
    return index
}

const fruits = [ 'apple', 'mango', 'banana', 'mango', 'orange' ]

console.log(fruits._lastIndexOf('mango'))

// Output
// 3

• here only difference in code is we store the index whenever we get a match and then return after the loop is complete.