successor method

utils.js

export function successor(prefix) {

	let limit = prefix;
	
  while ( limit.length > 0 ) {

		const index = limit.length - 1;
		
    if ( limit[index] === '\xff' )
      limit = limit.slice(0, -1)
    else {
      limit = limit.substr(0, index) + String.fromCharCode(limit.charCodeAt(index) + 1)
      break
		}
		
	}
	
	return limit;
	
}