1.0.4 • Public • Published


    Build, test and release

    Our browser plugin is compatible with all modern browsers and IE9+ (polyfills required).

    This simple to use plugin helps you to get started quickly, allowing you to add postcode lookup to any address form in minutes.


    See @firstclasspostcodes/plugin for detailed usage, guides and examples.


    Add the plugin directly into your HTML with:

    <script src="https://js.firstclasspostcodes.com/plugin/v1.0.4.js"></script>

    Note on older browsers: You will need to use a polyfill service, the following example covers all of the required language features:

    <script src="https://polyfill.io/v3/polyfill.min.js?features=Promise%2Cfetch%2CObject.assign%2Cdefault"></script>

    Add the client library

    We do not bundle the client library with the plugin, so you will need to add a separate script tag to require the JS client:

    <script src="https://js.firstclasspostcodes.com/v1.5.1.js"></script>

    All JS library versions are available on the @firstclasspostcodes/js releases page.


    Where the libary is loaded on pages including sensitive information, we recommend using the Subresource Integrity security feature.

    Every version of the library is accompanied by an SRI hash file, the hash can be accessed directly using:

    $ curl https://js.firstclasspostcodes.com/plugin/v1.0.4.sri.txt # => "sha256-45tfd... sha384-43567ytr..." 

    You can then update the above <script> tag, adding the integrity attribute:

    <script src="https://js.firstclasspostcodes.com/plugin/v1.0.4.js"
            integrity="sha256-45tfd... sha384-43567ytr..."


    You need to configure the plugin to use your API Key which is available on your dashboard. Below is a minimal working setup, you can see more guides on how to better customise the plugin by viewing the documentation.

    <div id="postcode-lookup-form"></div>
        <label for="address-line-1">Address Line 1</label>
        <input name="address-line-1" type="text" data-address-line1 />
        <label for="address-line-2">Address Line 2</label>
        <input name="address-line-2" type="text" data-address-line2 />
        <label for="county">County</label>
        <input name="county" type="text" data-address-locality />
        <label for="city">City</label>
        <input name="city" type="text" data-address-county />
        <label for="postcode">Postcode</label>
        <input name="postcode" type="text" data-address-postcode />
          apiKey: '111111111111',


    The plugin can be initialized with the same configuration overrides that you can supply for the @firstclasspostcodes/js library using the apiOverrides property (an example of its usage is below).

    If you're running the mock service docker container. You can configure the plugin to talk to it using the example below:

          apiKey: '111111111111', 
          apiOverrides: {
            endpoint: 'http://localhost:2345',


    Whilst not necessarily required, if you need to listen to events happening we support attaching event handlers for the following:

    Event Name Description
    address Fired when an address is selected from the drop down. Contains the address detail.

    Events are fired using the CustomEvent class. Event properties are included inside the .detail property.

    Event handlers can be attached using the following:

      var plugin = FirstclasspostcodesPlugin(...);
      plugin.$on('address', (event) => {


    Our plugin is built using Svelte and an instantiated component is returned from the FirstclasspostcodesPlugin() function. Therefore, the returned object is compatible with the documented Client-side component API.


    npm i @firstclasspostcodes/plugin

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    • davidkelley