The Preview feature enables you to view your static and dynamic content. The menu options are configurable for both types of content:

  • Preview status options - Can be used for only static content (HTML, CSS, JS, and text). You can also easily preview static content on your mobile device using Project > QR Code for Preview URL to generate a QR code that you can scan with a QR reader on your device.

  • Preview dynamic options - To access files or services that are executed on the server (PHY, Ruby, Note, etc.), you need to use the right-most menu option.

    To access your server side application, enter This accesses your box over port 80, which is usedful if your corporate firewall blocks ports other than 80 and 443.

    word1-word2 is an automatically generated subdomain name for your Codio box. You can configure your application to listen on ports defined in the URL. However, Codio only supports a restricted range of ports. Refer to this section for more details (insert link).

Using preview

Using the Preview button lets you preview one or more web pages with a simple button press. Codio creates three default entries in the Preview menu automatically:

Preview Button
  • Project Index - this is the default file to run for your project. It can be set by right-clicking a file in the Filetree. This option should be used to preview static content (typicall HTML files). For PHP, Ruby, etc., you should use the Box URL option.

  • Current File - whichever code file currently has focus. This option should only be used to preview static content (typically HTML files). For PHP, Ruby, etc., you should use the Box URL option.

  • Box URL - Use this option for previewing PHP, Ruby, or other server side languages over HTTPS. Click here <insert link> for more information on configuring port access for HTTPS.

If you right-click a file in the file tree or the tab, you can also select Preview Static.

Preview Static

See Apache Password Basic Authentication and HTTP Authentication with PHP for examples.

Modifying the Preview menu

You can customize the Preview menu using one of the following methods:

  • From the Preview or Run drop-down menus, choose Configure.

  • Open the .codio file in the root of your project (if it exists).

To customize the Run and Preview buttons, modify this section of the .codio file:

// Configure your Run and Preview buttons here.

// Run button configuration
  "commands": {
        "Node version": "node --version"

// Preview button configuration
  "preview": {
        "Project Index (static)": "https://{{domain}}/{{index}}",
        "Current File (static)": "https://{{domain}}/{{filepath}}",
        "Box URL": "https://{{domain3000}}/"

You can insert tokens to customize the menus:

{{domain3000}} is replaced by the public url required to access your project over port 80, which is useful if your corporate firewall blocks ports other than 80 and 443.

{{domain}} is used along with the port number (default 3000) to your service. For example:

"Box URL": "https://{{domain3000}}/",

For details about the various tokens that can be inserted see Customize Run Button.


Static content is served over Port 80. To access dynamic content you may need to specify the appropriate Port.

Unsecure content error

Codio runs over a secure connection using HTTPS, therefore so does the inline preview. If your code references an external resource such as a script, font, or image, a browser error may occur indicating that there is some form of unsecure or mixed content because you are running in a mixed HTTP/HTTPS mode. This may occur when you reference something similar to the following:

<script src="">

The error is caused by a restriction of the browser and cannot be easily modified. It is intended to protect you from insecure content.

To avoid this from occuring:

  • Modify your external references to use HTTPS.

  • Modify your references to use the ‘current protocol’ by including ‘//’ without http or https, so <script src=”//”>.

You can also download the external file to the Codio project and then reference it.