Array Slice For pagination

/**
	 * This function will return array start and end index
	 * which can be use in array slice Method
	 * @param {int} pageNumber 
	 * @param {int} posts_per_page
	 * Logic Behind it	
	 * 5
		0	0	5
		1	5	10
		2	10	15
		3	15	20
		arr.slice(0,5)
		(5) [1, 2, 3, 4, 5]
		arr.slice(5,10)
		(5) [6, 7, 8, 9, 10]
		arr.slice(10,15)
		(5) [11, 12, 13, 14, 15]
		arr.slice(15,20)
		(5) [16, 17, 18, 19, 20] 
	 */
	returnStartEndIndex(pageNumber, posts_per_page) {
		pageNumber = pageNumber || this.state.currentPage;
		posts_per_page = posts_per_page || this.state.posts_per_page;

		let startIndex = posts_per_page * pageNumber;
		let endIndex = startIndex + posts_per_page;

		return [ startIndex, endIndex ];
	}