Edit in JSFiddle

    let isMinified;
    let lineEndIndex = 0;
    let lineStartIndex = 0;
    let lines = 0;
    let indentCount = 0;

    // Strip comments.
    aText = aText.replace(/\/\*[\S\s]*?\*\/|\/\/(.+|\n)/g, "");

    while (lines++ < SAMPLE_SIZE) {
      lineEndIndex = aText.indexOf("\n", lineStartIndex);
      if (lineEndIndex == -1) {
         break;
      }
      if (/^\s+/.test(aText.slice(lineStartIndex, lineEndIndex))) {
        indentCount++;
      }
      lineStartIndex = lineEndIndex + 1;
    }
    isMinified = ((indentCount / lines ) * 100) < INDENT_COUNT_THRESHOLD;