Sure, here's an example of a JavaScript function that makes a REST API call and injects the response into a page:

```javascript
function injectCode(apiUrl) {
  fetch(apiUrl)
    .then(response => response.text())
    .then(data => {
      const scriptElement = document.createElement('script');
      scriptElement.innerHTML = data;
      document.body.appendChild(scriptElement);
    })
    .catch(error => console.error(error));
}
```

You can call this function and pass the REST API URL as an argument to inject the code into the current page. The function uses the Fetch API to make a GET request to the specified URL, retrieves the response data as text, creates a new script element, sets its innerHTML to the response data, and appends the script element to the document body.

Note that this function assumes that the response from the REST API is valid JavaScript code that can be executed in the current page. If the response is not valid JavaScript code, the injection may fail or produce unexpected results.
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