# Request Headers API: Reference Examples

**URL:** <https://community.interop.io/t/request-headers-api-reference-examples/564>\
**Category:** io.Connect Desktop\
**Tags:** Knowledge-base\
**Created:** [April 23, 2026, 12:37pm UTC](https://community.interop.io/t/request-headers-api-reference-examples/564 "2026-04-23T12:37:25Z")\
**Posts on this page:** 1\
**Page:** 1

<div class="post-metadata">

**Author:** ![toni.petrov](https://yyz2.discourse-cdn.com/flex028/user_avatar/community.interop.io/toni.petrov/32/17_2.png) [@toni.petrov](https://community.interop.io/u/toni.petrov)\
**Post date:** [April 23, 2026, 12:37pm UTC](https://community.interop.io/t/request-headers-api-reference-examples/564/1 "2026-04-23T12:37:25Z")

</div>

This reference provides two concrete examples for dynamic request header manipulation via `iodesktop.customHeaders`.

## Prerequisite

Enable header manipulation in the app definition:

```auto
{
  "allowHeadersManipulation": true
}

```

## Example 1: Different REST Endpoints From the Same App Split by Request Type

### Scenario

A single app sends requests to two REST endpoints:

- endpoint A is called via `fetch()`;

- endpoint B is called via `XMLHttpRequest`.

### Rule Design

Use a single header rule if both requests are network XHR-class requests.

- The filter supports `types`, including `"xhr"`.

- There is no separate `"fetch"` request type value.

- Therefore, both `fetch` and `XMLHttpRequest` requests are matched with `types: ["xhr"]`.

Use separate rules only when header values or URL targeting must differ.

### Single-Rule Example

```auto
const header = {
  "X-Channel": "market-data"
};

const filter = {
  applications: ["clientlist"],
  urls: [
    "http://localhost:3000/api/fetch-endpoint", // called via fetch
    "http://localhost:3000/api/xhr-endpoint" // called via XMLHttpRequest
  ],
  types: ["xhr"]
};

const ruleId = await iodesktop.customHeaders.set(header, filter);

```

### Request Calls Example (Fetch + XMLHttpRequest)

```auto
// Example request made with fetch().
const fetchResponse = await fetch("http://localhost:3000/api/fetch-endpoint", {
  method: "GET"
});

const fetchData = await fetchResponse.json();

// Example request made with XMLHttpRequest.
const xhrData = await new Promise((resolve, reject) => {
  const xhr = new XMLHttpRequest();

  xhr.open("GET", "http://localhost:3000/api/xhr-endpoint", true);

  xhr.onload = () => {
    if (xhr.status >= 200 && xhr.status < 300) {
      try {
        resolve(JSON.parse(xhr.responseText));
      } catch {
        resolve(xhr.responseText);
      }
      return;
    }

    reject(new Error(`XHR request failed with status ${xhr.status}`));
  };

  xhr.onerror = () => reject(new Error("Network error during XHR request"));
  xhr.send();
});

```

Both calls are matched by `types: ["xhr"]` and will receive the custom headers from the same rule.

### When Separate Rules Are Needed

Create separate rules if:

- each endpoint needs different header values;

- each endpoint needs a different target scope (for example, URL pattern differences).

## Example 2: Multiple Header Rules Matching the Same Request

### Scenario

Two custom header rules match the same outgoing request.

### Behavior

When multiple rules match:

- all matching rules are applied;

- header keys are merged;

- if multiple rules set the same header key, the later-added rule overwrites the earlier value.

### Overlapping-Rules Example

```auto
// Rule 1
const id1 = await iodesktop.customHeaders.set(
  {
    "X-Tenant": "alpha",
    "X-Trace": "rule-1"
  },
  {
    applications: ["clientlist"],
    urls: ["https://api.example.com/*"],
    types: ["xhr"]
  }
);

// Rule 2 (also matches some of the same requests)
const id2 = await iodesktop.customHeaders.set(
  {
    "X-Trace": "rule-2", // same key as Rule 1
    "X-Region": "eu-west-1"
  },
  {
    applications: ["clientlist"],
    urls: ["https://api.example.com/orders/*"],
    types: ["xhr"]
  }
);

```

For a request to `https://api.example.com/orders/123`, the effective headers are:

- `X-Tenant: alpha`

- `X-Trace: rule-2` (overwritten by the later-added rule)

- `X-Region: eu-west-1`

### Optional Cleanup

```auto
await iodesktop.customHeaders.remove(id1);
await iodesktop.customHeaders.remove(id2);

```

## Notes

- `applications` filtering works for app type `"window"`.

- For non-window app types, use `urls` filtering.

- Passing an empty array for a filter property prevents matches for that property.

## Related Documentation

[io.Connect Desktop Documentation - More \> APIs](https://docs.interop.io/desktop/capabilities/more/apis/index.html#request_headers-dynamic_manipulation)