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AngularJS directive to test the strength of a password

Recently I’m working a lot with AngularJS and validation of forms, and maybe you have noticed this (see: form validation in AngularJS, and validate password in AngularJS).

The topic of today is not strictly related to form validation, but, like also the form validation, it could be useful to improve the form usability, and even the user’s data security. Always more frequently during the registration on a new website, I happen to see that the password that I chose is judged, and I’m informed about its strength value.

In this post I will show how to code an AngularJS directive to measure the strength of the password typed by the users.

Live demo

See the demo by Bruno Scopelliti (@brunoscopelliti) on CodePen.

The code

Before start digging into the code, I need a short premise.

The goal of this post is to show how you could write an AngularJS directive to test the strength of a password, not to give the best algorithm to test the strength of a password; indeed in my opinion the algorithm I implemented is really bad. You could find a better solution here.

As always, let’s start from the markup:

<input type="password" ng-model="pw" name="pw" id="pw" />
<ul id="strength" check-strength="pw"></ul>

As you can see the directive checkStrength expects as parameter the id of the input field that will contain the password typed by the user. Inside the directive is positioned the most important part of the code. I created the strength object, to manage the tasks:

  • compute of strength of the password
  • bind the level of strength to a color

How this code really looks like is not very important to understand the working of the directive; for this reason I will show here only the backbone of the object. However you could find the complete example on Codepen.

var strength = {
  colors: ["#F00", "#F90", "#FF0", "#9F0", "#0F0"],
  mesureStrength: function (p) {
    // return the evaluated strength of the password
  getColor: function (s) {
      return an object with the format:
      obj = { idx: strengthLevel, col: 'color' }

Finally the directive:

angular.module("myApp.directives", [])
  .directive("checkStrength", function () {
    return {
      replace: false,
      restrict: "EACM",
      link: function (scope, iElement, iAttrs) {

        var strength = {

        scope.$watch(iAttrs.checkStrength, function () {
          if (scope.pw === "") {
            iElement.css({ "display": "none"  });
          } else {
            var strength = strength.mesureStrength(scope.pw);
            var c = strength.getColor(strength);
            iElement.css({ "display": "inline" });
              .css({ "background": "#DDD" })
              .slice(0, c.idx)
              .css({ "background": c.col });

      template: ""

In this case there are two important things to notice:

  • In the directive definition object is defined the template property. This allows to add additional elements to the DOM. In this specific case, five more list items are added.

  • In the link function is used the api method $watch to register callback to be executed whenever the password changes.