Return value in function from a promise block

You could first get all the texts with promise.map and then get the position with indexOf :

var map = webdriver.promise.map;

var findCreatedTask = function() {
    var elems = driver.findElements(By.xpath("//div[@id='Tasks_Tab']//div[@class="task-title"]"));
    return map(elems, elem => elem.getText()).then(titles => {
      var position = titles.indexOf("testName") + 1;
      return "//div[@id='Tasks_Tab'][" + position + "]";
    });
}