Example: How to insert html tags efficiently.

insert-html-tag-to-message.js

/*
 * Below example converts from "1hoge2foo3bar456" to "1<b>hoge</b>2<b>foo</b>3<b>bar</b>456".
 */

const tag = "b"; // <b> tag.

// sample message
let msg = "1hoge2foo3bar456";

/* insert point indice */
const insPoints = [
  [1, 4], // "hoge"
  [6, 8], // "foo"
  [10, 12], // "bar"
];

let insPointsIdx = 0;

const buf = [];

msg.split("").forEach((s, idx) => {
  // console.log(s, idx);
  const insPoint = insPoints[insPointsIdx];
  if(!insPoint) return buf.push(s);
  if (idx === insPoint[0]) {
    buf.push(`<${tag}>`);
  }
  buf.push(s);
  if (idx === insPoint[1]) {
    buf.push(`</${tag}>`);
    insPointsIdx += 1;
  }
});

console.log("buf:", buf.join("")); // buf: 1<b>hoge</b>2<b>foo</b>3<b>bar</b>456