transpose-chars.js

// ============================================================ //
// Inside of PRESERVE area.
// ============================================================ //

function transposeSubString(input, beg, end, to) {
    let txt = input.value;

    let head  = txt.slice(0, beg);
    let left  = txt.slice(beg, end);
    let right = txt.slice(end, to);
    let tail  = txt.slice(to);

    let {scrollTop, scrollLeft} = input;

    input.value          = head + right + left + tail;
    input.selectionStart = input.selectionEnd = txt.length - tail.length;

    if (scrollTop === scrollLeft === 0)
        command.inputScrollSelectionIntoView(input);
    else
        input.scrollTop = scrollTop, input.scrollLeft = scrollLeft;
}

function transposeChars(ev, arg) {
    let input = ev.originalTarget;
    let begin = input.selectionEnd - 1;
    let end   = begin + 1;
    let to    = end + (typeof arg === 'number' ? Math.max(arg, 1) : 1);
    transposeSubString(input, begin, end, to);
}

ext.add("transpose-chars", transposeChars, "Interchange characters around point");

// ============================================================ //
// Bottom of the .keysnail.js
// ============================================================ //

key.setEditKey("C-t", function (ev, arg) { ext.exec("transpose-chars", arg, ev); }, "Transpose chars", true);