Separate array elements using correct English grammar.

arrayElementsSeparator.js

const fruits = ['apple', 'banana', 'orange', 'jackfruit']
let separatedList = ''

fruits.forEach((fruit, index) => {
	const isSeparated = fruits.length > 1 && (index + 1 < fruits.length)
  const isCommaSeparated = isSeparated && (index + 2 < fruits.length)
  const isAmpersandSeparated = isSeparated && (index + 2 === fruits.length)

	separatedList = separatedList + fruit

	if (isCommaSeparated)
		separatedList = separatedList + ', '

	if (isAmpersandSeparated)
		separatedList = separatedList + ' and '
})

This gist depicts how to create a grammatically correct list* from an array of strings.

*abides by the Oxford comma.

Example:
let fruits = ['apple', 'banana', 'orange', 'jackfruit']
will output apple, banana, orange, and jackfruit

let fruits = ['apple', 'banana']
will output apple and banana

let fruits = ['apple']
will output apple