update textarea value, but keep cursor position

Question

I have an html textarea that will be updated periodically via javascript.

when I do this:

$("#textarea").val(new_val);

The cursor moves to the end of the text.

I would like to update the text without changing the cursor position. Also, if the user has a range of text selected, the highlight should be preserved.

Answer 1

With Java-Diff-Utils one can update the text and have the cursor position adapted "intelligently".

import com.github.difflib.DiffUtils;
import com.github.difflib.patch.AbstractDelta;

private void setTextAndUpdateCaretPosition(TextInputControl textInputControl, String newText) {
    int lastCaretPosition = textInputControl.getCaretPosition();
    String oldText = textInputControl.getText();
    textInputControl.setText(newText);
    if (oldText == null) {
        return;
    }
    if (newText == null) {
        return;
    }
    if (oldText.equals(newText)) {
        return;
    }
    // This is a special case when the text is set to a new value
    // In this case, we want to adjust the caret position
    List<String> oldValueCharacters = Arrays.asList(oldText.split(""));
    List<String> newValueCharacters = Arrays.asList(newText.split(""));
    List<AbstractDelta<String>> deltaList = DiffUtils.diff(oldValueCharacters, newValueCharacters).getDeltas();
    AbstractDelta<String> lastDelta = null;
    for (AbstractDelta<String> delta : deltaList) {
        if (delta.getSource().getPosition() > lastCaretPosition) {
            break;
        }
        lastDelta = delta;
    }
    if (lastDelta == null) {
        // Change happened after current caret position
        // Thus, simply restore the old position
        textInputControl.positionCaret(lastCaretPosition);
    } else {
        int offset = lastDelta.getTarget().getPosition() - lastDelta.getSource().getPosition();
        switch (lastDelta.getType()) {
            case DELETE:
                offset -= lastDelta.getSource().size();
                break;
            case INSERT:
                offset += lastDelta.getTarget().size();
                break;
            case CHANGE:
                offset += lastDelta.getTarget().size() - lastDelta.getSource().size();
                break;
            default:
                break;
        }
        int newCaretPosition = lastCaretPosition + offset;
        textInputControl.positionCaret(newCaretPosition);
    }
}

Full source in real world application: https://github.com/JabRef/jabref/blob/3401b71fffeca4178ba8ffabadfac7a65a737915/src/main/java/org/jabref/gui/fieldeditors/FieldEditorFX.java#L41

Answer 2

A decade later but this is what I came up with for replacing items in a textarea. Some additional handling is needed to adjust the caret or selection when replacing with longer or shorter text.

// find and replace in textarea while preserving caret and selection
function replaceText(el, findText, replaceWithText) {
    var text = el.value;
    var selectionStart = 0;
    var selectionEnd = 0;
  // only support modern browsers for preserving caret and selection
    if (el.setSelectionRange) {
        selectionStart = el.selectionStart;
        selectionEnd = el.selectionEnd;
    }
    var start = 0;
    while ((start = text.indexOf(findText, start)) > -1) {
        var end = start + findText.length;
        text = text.substr(0, start) + replaceWithText + text.substr(end);
        if (selectionStart < end) {
            selectionStart = Math.min(selectionStart, start + replaceWithText.length);
        } else {
            selectionStart = selectionStart + replaceWithText.length - (end - start);
        }
        if (selectionEnd < end) {
            selectionEnd = Math.min(selectionEnd, start + replaceWithText.length);
        } else {
            selectionEnd = selectionEnd + replaceWithText.length - (end - start);
        }
        start += replaceWithText.length;
    }
  // don't do anything unless we need to (otherwise destroys undo)
    if (el.value != text) { 
        el.value = text;
        if (el.setSelectionRange) {
            el.selectionStart = selectionStart;
            el.selectionEnd = selectionEnd;
        }
    }
}

Place caret on or after the word LONGER, or select some text after or including it:
<br />
<textarea id='t'>Here is
some LONGERtext
to replace</textarea>
<br />
<input type="button" onclick="replaceText(document.getElementById('t'),'LONGER',''); document.getElementById('t').focus();" value="remove word LONGER" />

Answer 3

Here is a pair of functions that get and set the selection/caret position in a text area in all major browsers.

Note: if you don't need to support IE <= 8, just use the selectionStart and selectionEnd properties (MDN). All of the complicated code below is just there to support old versions of IE.

function getInputSelection(el) {
    var start = 0, end = 0, normalizedValue, range,
        textInputRange, len, endRange;

    if (typeof el.selectionStart == "number" && typeof el.selectionEnd == "number") {
        start = el.selectionStart;
        end = el.selectionEnd;
    } else {
        range = document.selection.createRange();

        if (range && range.parentElement() == el) {
            len = el.value.length;
            normalizedValue = el.value.replace(/\r\n/g, "\n");

            // Create a working TextRange that lives only in the input
            textInputRange = el.createTextRange();
            textInputRange.moveToBookmark(range.getBookmark());

            // Check if the start and end of the selection are at the very end
            // of the input, since moveStart/moveEnd doesn't return what we want
            // in those cases
            endRange = el.createTextRange();
            endRange.collapse(false);

            if (textInputRange.compareEndPoints("StartToEnd", endRange) > -1) {
                start = end = len;
            } else {
                start = -textInputRange.moveStart("character", -len);
                start += normalizedValue.slice(0, start).split("\n").length - 1;

                if (textInputRange.compareEndPoints("EndToEnd", endRange) > -1) {
                    end = len;
                } else {
                    end = -textInputRange.moveEnd("character", -len);
                    end += normalizedValue.slice(0, end).split("\n").length - 1;
                }
            }
        }
    }

    return {
        start: start,
        end: end
    };
}

function offsetToRangeCharacterMove(el, offset) {
    return offset - (el.value.slice(0, offset).split("\r\n").length - 1);
}

function setInputSelection(el, startOffset, endOffset) {
    if (typeof el.selectionStart == "number" && typeof el.selectionEnd == "number") {
        el.selectionStart = startOffset;
        el.selectionEnd = endOffset;
    } else {
        var range = el.createTextRange();
        var startCharMove = offsetToRangeCharacterMove(el, startOffset);
        range.collapse(true);
        if (startOffset == endOffset) {
            range.move("character", startCharMove);
        } else {
            range.moveEnd("character", offsetToRangeCharacterMove(el, endOffset));
            range.moveStart("character", startCharMove);
        }
        range.select();
    }
}

When you change the textarea's value, first save the selection, then restore it afterwards:

var t = document.getElementById("textarea");
var sel = getInputSelection(t);
t.value = some_new_value;
setInputSelection(t, sel.start, sel.end);