maskify.js

function maskify(credit_card){
	let credit_card_string = credit_card.toString()
	let card_length = credit_card_string.length
	if(card_length < 6){
		return credit_card_string
	}
	let card_characters_array = credit_card.split('')
	let masked = card_characters_array.map( (char,index) =>{
	
		if(index === 0 || index > card_length -4){
			return char
		}
		if(Number.isInteger(parseInt(char))){
			return '#'
		}
		return char
	})
	return masked.join('')
}
console.log(maskify('3454534A'))