JS callback-promise combined pattern : good or bad practice?

I'm writting some library that will provide asynchronous methods, I want the user to be able to use classic callbacks or modern promises depending on its own preference.

I was coding two versions of each methods by naming them 'myMethod' and 'myMethodPromise', then a thought came accross my mind :

Why not coding a method that combines the two patterns?

One single method with an optional callback argument, if not provided then the method returns a promise instead of calling the callback.

Would it be a good practice?

// Promise-callback combined pattern method
function myAsyncMethod ( callback = null ) {
        if(callback) {
        var result = "xxx";
        // Do something...
                callback(result);
    } else {
          return(new Promise((res, rej) => {
        var result = "xxx";
        // Do something...
        res(result);
    }))
  }
}

// Usage with callback
myAsyncMethod((result)=>document.getElementById('callbackSpan').innerHTML = result);
// or with promise
myAsyncMethod().then((result) => document.getElementById('promiseSpan').innerHTML = result);

<p>
Result with callback : <span id="callbackSpan"></span>
</p>
<p>
Result with promise : <span id="promiseSpan"></span>
</p>

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