Content filters

Content filters are a way to manipulate the response before it is sent to the client.

This can be useful for a variety of purposes such as:

• Injecting additional content or scripts into the response
• Using regex to replace content in the response
• Blocking access to certain content
• Implementing security measures, such as preventing markup containing reference to a compromised third-party service

Getting started

To begin, create a new content filter in the QuantCDN dashboard from the “Compute > Content filters” section.

The code editor will open in the right-hand pane. By default, the editor will contain a basic example of a content filter.

The response is manipulated as it is streamed to the client, so you can alter the response in real-time on chunks of data.

Tip

Content filters only work on content when the response is HTML (e.g the response header is Content-Type: text/html).

Examples

Replacing content in the response

The following example shows how to replace all instances of the word “example” with the word “test”.

class contentFilter extends TransformStream {
    constructor(event) {
        super({
            transform: (chunk, controller) => {
                const chunkStr = this.textDecoder.decode(chunk);
                const transformedChunkStr = chunkStr.replace(/example/g, 'test');
                const outputBytes = this.textEncoder.encode(transformedChunkStr)
                controller.enqueue(outputBytes);
            },
        });
        this.textEncoder = new TextEncoder();
        this.textDecoder = new TextDecoder();
    }
}

Injecting JavaScript into the response

The following example shows how to inject a custom script into the response, before the closing </body> tag.

class contentFilter extends TransformStream {
    constructor(event) {

        const clientCountryCode = event.request.headers.get('client-geo-country') ?? 'XX'

        super({
            transform: (chunk, controller) => {
                const chunkStr = this.textDecoder.decode(chunk);
                // Example alert script with the client country code
                const exampleScript = `<script>alert('${clientCountryCode}');</script>`
                const transformedChunkStr = chunkStr.replace('</body>', exampleScript + '</body>');
                const outputBytes = this.textEncoder.encode(transformedChunkStr)
                controller.enqueue(outputBytes);
            },
        });
        this.textEncoder = new TextEncoder();
        this.textDecoder = new TextDecoder();
    }
}

Applying a content filter to a route

To apply a content filter to the route, you can add a new Page Rule to apply to specific domains or routes (or any other supported condition).