5 Puzzling JavaScript Bugs

by elena vilchik|

    Let’s play a game: you see a piece of JavaScript and try to find a bug there. It could be for example a dead code, a runtime error or some unexpected behaviour.

    By the way these snippets are not figment of my poor mind, but are the real code from some open-source projects simplified for the sake of the game.

    The answers are just after code, don’t cheat!

    Bug #1

    function createPopups(accessKey, text, relatedFieldId) {
      var formField = relatedFieldId !== "#" ? relatedFieldId : null;
      if (accessKey && text === accessKey) {
        createPopup("label", accessKey);

    Answer (line 8)

    Look at the last line of the function: we are accessing an element with index zero on formField. This variable is initialised with either relatedFieldId or null. In the latter case the index access will crash the execution.

    Bug #2

    function calculateOpenState() {
      var rank = 0;
      var today = new Date();
      var start = new Date(this.dateStarted);
      var end = new Date(this.dateCompleted);
      if (start <= today && today <= end) {
        rank = 5;
      } else {
        if (today < start) {
          rank = 1;
        } else if (today > end) {
          rank = 2;
        } else {
          rank = 3;
      return { state: STATES[rank], rank: rank };

    Answer (line 15)

    The code rank = 3; is never executed as the condition on line 12 today > end is always evaluates to true.

    Bug #3

    function roundMinAndMax(max, min, units, roundingUnit) {
      if (this.alwaysShowZero()) {
        max = 0;
        if (this.useIntegers()) {
          min = max - roundingUnit * units;
        } else {
          min = max - Math.ceil(roundingUnit * units * 100000) / 100000;
      this.dataMaximum = max;
      this.dataMinimum = min;

    Answer (line 5)

    Look at Math.ceil, it’s used 2 times in this function, but one time the return value is not used.

    Bug #4

    function hideAll(name, config) {
      if (name && Y.Transition) {
        if (typeof config === "function") {
          config = null;
        if (!name.push) {
          if (typeof config === "function") {
            config = name;
          name = "HIDE_TRANSITION";
        this.transition(name, config);
      } else {
      return this;

    Answer (line 9)

    The second time we evaluate a condition typeof config === ‘function’(line 8), this condition will be always false, as on line 3 there is the same check, and when it’s true the value of config changed to null. That way we have a dead code at line 9.

    Bug #5

    function comparator(a, b) {
      var aLastAccessTime = a.get("lastAccessTime");
      var bLastAccessTime = b.get("lastAccessTime");
      if (aLastAccessTime !== bLastAccessTime) {
        return bLastAccessTime - aLastAccessTime;
      } else if (aLastAccessTime && !bLastAccessTime) {
        return -1;
      } else if (!aLastAccessTime && bLastAccessTime) {
        return 1;
      return 0;

    Answer (lines 7 and 9)

    Both conditions aLastAccessTime && !bLastAccessTime and !aLastAccessTime && bLastAccessTime are never true and a code next to them is never executed. We reach these conditions only if the first condition aLastAccessTime !== bLastAccessTime is false, thus aLastAccessTime and bLastAccessTime are strictly equal.

    That’s All Folks!

    So how many bugs did you find by yourself and how long did it take for you? The point of all of that is to show how hard sometimes is to manually find an inconsistency or some problem in the code. And of course developers should never underestimate the power of automated tools for increasing the quality and robustness of code, such as linters, testing frameworks, and continuous inspection.

    All of these bugs were found by SonarJS, a static code analyser for JavaScript. In this tool data-flow analysis is used to find anomalies in the program execution without actually executing the code. SonarJS runs as an IDE plugin within SonarLint and it also runs online within SonarCloud, a service for continuous code quality.