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Property validation in Polymer

Asked By: Anonymous

In a normal JavaScript class I can use constructor and setters to ensure that my object is never passed invalid values and thus never enters an invalid state.

How would I do this in Polymer?

Here is why I am asking. I have a bug resulting from one of my component properties having an invalid value. I’m not sure how it became invalid. Normally, my first step would be to ensure that it’s not being passed bad values. I would do this by using constructors and setters.

With Polymer being a layer of magic on top of Plain Old JavaScript, I’m not sure how to approach this problem.


Solution

Answered By: Anonymous
  • Option 1: Create a property observer that logs the stacktrace (with console.trace()) when an invalid value is set. While this option doesn’t prevent the invalid value, it helps you find the bug.
    _x000D_

    _x000D_

    <head>_x000D_
      <base href="https://polygit.org/polymer+1.6.0/components/">_x000D_
      <a href="http://webcomponentsjs/webcomponents-lite.min.js">http://webcomponentsjs/webcomponents-lite.min.js</a>_x000D_
      <link rel="import" href="polymer/polymer.html">_x000D_
      <link rel="import" href="paper-input/paper-input.html">_x000D_
    </head>_x000D_
    <body>_x000D_
      <x-foo></x-foo>_x000D_
    _x000D_
      <dom-module id="x-foo">_x000D_
        <template>_x000D_
          <!-- paper-input has better methods of input validation, but this_x000D_
               is intended for demonstrating property validation where the_x000D_
               value can be set in any manner (not just through inputs) -->_x000D_
          <paper-input label="Enter even number" value="{{foo}}"></paper-input>_x000D_
          <div>valid: [[_isValid]]</div>_x000D_
        </template>_x000D_
        <script>_x000D_
        // HTMLImports.whenReady() in index.html for demo only_x000D_
        HTMLImports.whenReady(function() {_x000D_
          "use strict";_x000D_
    _x000D_
          Polymer({_x000D_
            is: 'x-foo',_x000D_
            properties : {_x000D_
              foo: {_x000D_
                type: Number,_x000D_
                value: 2,_x000D_
                observer: '_fooChanged'_x000D_
              },_x000D_
              _isValid: {_x000D_
                type: Boolean,_x000D_
                value: false_x000D_
              }_x000D_
            },_x000D_
            _x000D_
            _fooChanged: function(foo) {_x000D_
              this._isValid = foo % 2 == 0;_x000D_
              if (!this._isValid) {_x000D_
                console.trace('invalid value');_x000D_
              }_x000D_
            }_x000D_
          });_x000D_
        });_x000D_
        </script>_x000D_
      </dom-module>_x000D_
    </body>

    _x000D_

    _x000D_

    _x000D_

    codepen

  • Option 2: Proxy the property through a validating computed property.
    _x000D_

    _x000D_

    <head>_x000D_
      <base href="https://polygit.org/polymer+1.6.0/components/">_x000D_
      <a href="http://webcomponentsjs/webcomponents-lite.min.js">http://webcomponentsjs/webcomponents-lite.min.js</a>_x000D_
      <link rel="import" href="polymer/polymer.html">_x000D_
      <link rel="import" href="paper-input/paper-input.html">_x000D_
    </head>_x000D_
    <body>_x000D_
      <x-foo></x-foo>_x000D_
    _x000D_
      <dom-module id="x-foo">_x000D_
        <template>_x000D_
          <!-- paper-input has better methods of input validation, but this_x000D_
               is intended for demonstrating property validation where the_x000D_
               value can be set in any manner (not just through inputs) -->_x000D_
          <paper-input label="Enter even number" type="number" value="{{foo}}"></paper-input>_x000D_
          <span>nearest even: [[_foo]]</span>_x000D_
        </template>_x000D_
        <script>_x000D_
        HTMLImports.whenReady(() => {_x000D_
          "use strict";_x000D_
    _x000D_
          Polymer({_x000D_
            is: 'x-foo',_x000D_
            properties : {_x000D_
              foo: {_x000D_
                type: Number,_x000D_
                value: 2_x000D_
              },_x000D_
              _foo: {_x000D_
                computed: '_computeEvenFoo(foo)'_x000D_
              }_x000D_
            },_x000D_
            _x000D_
            _computeEvenFoo: function(foo) {_x000D_
              const num = Number(foo);_x000D_
              const isValid = num % 2 == 0;_x000D_
              if (!isValid) {_x000D_
                console.trace('overriding invalid value');_x000D_
                return num + 1;_x000D_
              }_x000D_
              return foo;_x000D_
            }_x000D_
          });_x000D_
        });_x000D_
        </script>_x000D_
      </dom-module>_x000D_
    </body>

    _x000D_

    _x000D_

    _x000D_

    codepen

In addition to (or instead of) logging the stacktrace, you could setup a breakpoint in DevTools (with Chrome, Firefox, Safari, etc.) to catch the bug as it occurs via your observer/computed property.

techinplanet staff

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