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.

Tim Down · Accepted Answer · 2016-06-09T09:29:10.483

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);

Does the `setInputSelection` function work only in IE? I'm asking because the `createTextRange` method does not exist (for input elements) in the other browsers, so that an error is thrown. — Šime Vidas, Nov 16 '10 at 14:41
I created a demo using your code. The code is here: http://vidasp.net/js/selection.js and the demo is here: http://vidasp.net/tinydemos/select-demo.html The demo works flawlessly in IE9 beta, Chrome and Safari. However, there is an issue in Firefox: If you set the selection programmatically ( `selec.set(input, 10, 20);` it wont work **unless** you focus the text-box ( `input.focus();` ). In my demo, I am focusing the text-box after setting the selection - just to make it work in Firefox. Therefore, consider placing `el.focus()` at the end of the `setInputSelection` function. — Šime Vidas, Nov 16 '10 at 19:20
Is this still the best way to handle this? I came across this code in an older project and it seems really long/complicated for such a seemingly simple task — Marie, Apr 05 '16 at 19:01
@Marie: The complication is just there for IE <= 8. You can lose nearly all of it and just use `selectionStart` and `selectionEnd` if you don't need to support IE 8. — Tim Down, Apr 08 '16 at 09:53
One note - `typeof NaN === 'number'` comes out to `true`. This happens to not hit that boundary case, but in case anyone messes around with the code, they should know that it'd be stronger to handle that case. — knod, Sep 24 '19 at 12:02

Tom Davenport · Answer 2 · 2021-02-26T10:40:46.900

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" />

update textarea value, but keep cursor position

2 Answers2

Linked