Promise

Besides collections and objects for most document elements, the new JavaScript API implements asynchronous methods with callback functions. As all methods are asynchronous, it is important to wait for the return value which is solved using callbacks.

A callback is essentially a function that is executed after another function has finished executing.

The JavaScript method

&#9584; TX Text Control .NET Server &#9584; JavaScript API &#9584; Selection Object &#9584; getStart Method Gets the starting point of selected text. " data-tippy-interactive="true" data-tippy-placement="bottom" data-tooltip-width="wide"

>get Start of the Selection object has the following syntax:

Selection.getStart( callback, [ errorCallback]) It returns a RequestNumberCallback when successfully executed and an ErrorCallback when failed. The call of this method should look like this:

TXTextControl.selection.getStart( function (curSelStart) { // success console.log(curSelStart); }, function (errArg) { // error console.log(errArg); }, ); The above code returns safely the selection start index of the current input position. Now, what if we need to check the selection length value in the same function? JavaScript is an event driven language. That implies that Javascript is not waiting for a response before moving on. In the following function, we cannot be 100% certain that the value of the first function is returned first.

function GetValues() { TXTextControl.selection.getStart( function (curSelStart) { // success console.log(curSelStart); } );

TXTextControl.selection.getLength( function (curSelLength) { // success console.log(curSelLength); } ); } In order to achieve this, the second function should be called in the callback function of the first function:

Therefore, Promises and async functions can be used. The following function returns a range object with the start and end index of the current selection. The return value itself is a Promise that can be resolved and rejected .

function getSelectionRange() { return new Promise(resolve => { TXTextControl.selection.getStart(function (curSelStart) { TXTextControl.selection.getLength(function (curSelLength) {

var range = { start: curSelStart, end: curSelStart + curSelLength, };

resolve(range); }); }); }); } The calling function must be async in order to wait for Promises:

async function GetValues() { var range = await getSelectionRange(); console.log(range.start); console.log(range.end); } The keyword await marks the function to wait for a returned Promise, so that the return value can be used in further code in the same function.

TX Text Control .NET Server

DS Server

Server & Web

Windows Forms

WPF

ActiveX

Core Technologies

Blog

Core Technologies

Blog

Authors

Latest Posts

Support Resources

Documentation

Support

Getting Started

Other Resources

About Us

Industry events

Company

Contact

Newsletter

Legal

Sign in

Create account

Text Control Account

Promise

Post tagged with Promise in 2020

Server & Web

Windows Forms

WPF

ActiveX

Core Technologies

​

Latest Posts

​

Support

Getting Started

​

Other Resources

Company

Contact

Newsletter

Legal

Text Control Account

​

​

Promise

Post tagged with Promise in 2020 ​

March 2020

Post tagged with Promise in 2020