JSFiddle - React, Tailwind, and code Playground

JavaScript

var stringToHighlight = "ACGATCGATCGGATCGAGCGATCGAGCGATCGAT";
var lengthDelta = 0;

var highlightPositions = [[1,3], [8,10], [15,18]];

for (var highlight in highlightPositions) {
    var start = highlightPositions[highlight][0] + lengthDelta;
    var end = highlightPositions[highlight][1] + lengthDelta;
    
    var first = stringToHighlight.substring(0, start);
    var second = stringToHighlight.substring(start, end + 1);
    var third = stringToHighlight.substring(end + 1);
 
    stringToHighlight = first + "<b>" + second + "</b>" + third;
    lengthDelta += 9;
}

alert(stringToHighlight);