This function allows you to generate a fully-customizable pagination

customizablePaginationAlgorithm.js

function customizablePaginationAlgorithm(currentPage, totalPages, options = {}) {

	const {
		margin = 2,
		outerMargin = margin,
		afterFirst = outerMargin,
		beforeLast = outerMargin,
		innerMargin = margin,
		beforeCurrent = innerMargin,
		afterCurrent = innerMargin,
		showPageNumberOnSingleElementEllipsis = true,
		ellipsis = '\u2026'
	} = options;

	const result = [];

	const boundedAfterFirst = Math.min(1 + afterFirst, totalPages);

	for(let i = 1; i <= boundedAfterFirst; ++i) {
		result.push({ page: i, ellipsis: false, current: i === currentPage });
	}

	if(boundedAfterFirst === totalPages) {
		return result;
	}

	const boundedBeforeCurrent = Math.max(currentPage - beforeCurrent, boundedAfterFirst + 1);
	const boundedAfterCurrent = Math.min(currentPage + afterCurrent, totalPages);

	if(boundedBeforeCurrent > boundedAfterFirst + 1) {
		if(showPageNumberOnSingleElementEllipsis && boundedBeforeCurrent === boundedAfterFirst + 2) {
			result.push({ page: boundedAfterFirst + 1, ellipsis: false, current: false });
		} else {
			result.push({ page: ellipsis, ellipsis: true, current: false });
		}
	}

	for(let i = boundedBeforeCurrent; i <= boundedAfterCurrent; ++i) {
		result.push({ page: i, ellipsis: false, current: i === currentPage });
	}

	if(boundedAfterCurrent === totalPages) {
		return result;
	}

	const boundedBeforeLast = Math.max(totalPages - beforeLast, boundedAfterCurrent + 1);

	if(boundedBeforeLast > boundedAfterCurrent + 1) {
		if(showPageNumberOnSingleElementEllipsis && boundedBeforeLast === boundedAfterCurrent + 2) {
			result.push({ page: boundedAfterCurrent + 1, ellipsis: false, current: false });
		} else {
			result.push({ page: ellipsis, ellipsis: true, current: false });
		}
	}

	for(let i = boundedBeforeLast; i <= totalPages; ++i) {
		result.push({ page: i, ellipsis: false, current: i === currentPage });
	}

	return result;

}

Usage:

customizablePaginationAlgorithm(12, 20, {
    innerMargin: 3,
    outerMargin: 1,
    ellipsis: '...'
});

Output:

[
    {   page: 1,       ellipsis: false,   current: false   },
    {   page: 2,       ellipsis: false,   current: false   },
    {   page: '...',   ellipsis: true,    current: false   },
    {   page: 9,       ellipsis: false,   current: false   },
    {   page: 10,      ellipsis: false,   current: false   },
    {   page: 11,      ellipsis: false,   current: false   },
    {   page: 12,      ellipsis: false,   current: true    },
    {   page: 13,      ellipsis: false,   current: false   },
    {   page: 14,      ellipsis: false,   current: false   },
    {   page: 15,      ellipsis: false,   current: false   },
    {   page: '...',   ellipsis: true,    current: false   },
    {   page: 19,      ellipsis: false,   current: false   },
    {   page: 20,      ellipsis: false,   current: false   }
]