function update(el, headToCursor, cursorToTail) {
    const curr = el.value; // strA + strB1 + strC
    const next = headToCursor + (cursorToTail || ""); // strA + strB2 + strC
    const activeElement = document.activeElement;
    //  Calculate length of strA and strC
    let aLength = 0;
    let cLength = 0;
    while (aLength < curr.length && aLength < next.length && curr[aLength] === next[aLength]) {
        aLength++;
    }
    while (curr.length - cLength - 1 >= 0 &&
        next.length - cLength - 1 >= 0 &&
        curr[curr.length - cLength - 1] === next[next.length - cLength - 1]) {
        cLength++;
    }
    aLength = Math.min(aLength, Math.min(curr.length, next.length) - cLength);
    // Select strB1
    el.setSelectionRange(aLength, curr.length - cLength);
    // Get strB2
    const strB2 = next.substring(aLength, next.length - cLength);
    // Replace strB1 with strB2
    el.focus();
    if (!document.execCommand("insertText", false, strB2)) {
        // Document.execCommand returns false if the command is not supported.
        // Firefox and IE returns false in this case.
        el.value = next;
        // Dispatch input event. Note that `new Event("input")` throws an error on IE11
        const event = document.createEvent("Event");
        event.initEvent("input", true, true);
        el.dispatchEvent(event);
    }
    // Move cursor to the end of headToCursor
    el.setSelectionRange(headToCursor.length, headToCursor.length);
    activeElement.focus();
    return el;
}

function wrapCursor(el, before, after) {
    const initEnd = el.selectionEnd;
    const headToCursor = el.value.substr(0, el.selectionStart) + before;
    const cursorToTail = el.value.substring(el.selectionStart, initEnd) + (after || "") + el.value.substr(initEnd);
    update(el, headToCursor, cursorToTail);
    el.selectionEnd = initEnd + before.length;
    return el;
}

export { update, wrapCursor };
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