Get the bounding box coordinates of the actual text within an HTML input box

getTextBoundingRect.js

// http://stackoverflow.com/questions/6930578/get-cursor-or-text-position-in-pixels-for-input-element/7948715#7948715


function getTextBoundingRect(input, selectionStart, selectionEnd, debug) {
    // Basic parameter validation
    if (!input || !('value' in input)) return input;
    if (typeof selectionStart == "string") selectionStart = parseFloat(selectionStart);
    if (typeof selectionStart != "number" || isNaN(selectionStart)) {
        selectionStart = 0;
    }
    if (selectionStart < 0) selectionStart = 0;
    else selectionStart = Math.min(input.value.length, selectionStart);
    if (typeof selectionEnd == "string") selectionEnd = parseFloat(selectionEnd);
    if (typeof selectionEnd != "number" || isNaN(selectionEnd) || selectionEnd < selectionStart) {
        selectionEnd = selectionStart;
    }
    if (selectionEnd < 0) selectionEnd = 0;
    else selectionEnd = Math.min(input.value.length, selectionEnd);

    // If available (thus IE), use the createTextRange method
    if (typeof input.createTextRange == "function") {
        var range = input.createTextRange();
        range.collapse(true);
        range.moveStart('character', selectionStart);
        range.moveEnd('character', selectionEnd - selectionStart);
        return range.getBoundingClientRect();
    }
    // createTextRange is not supported, create a fake text range
    var offset = getInputOffset(),
        topPos = offset.top,
        leftPos = offset.left,
        width = getInputCSS('width', true),
        height = getInputCSS('height', true);

    // Styles to simulate a node in an input field
    var cssDefaultStyles = "white-space:pre;padding:0;margin:0;",
        listOfModifiers = ['direction', 'font-family', 'font-size', 'font-size-adjust', 'font-variant', 'font-weight', 'font-style', 'letter-spacing', 'line-height', 'text-align', 'text-indent', 'text-transform', 'word-wrap', 'word-spacing'];

    topPos += getInputCSS('padding-top', true);
    topPos += getInputCSS('border-top-width', true);
    leftPos += getInputCSS('padding-left', true);
    leftPos += getInputCSS('border-left-width', true);
    leftPos += 1; //Seems to be necessary

    for (var i = 0; i < listOfModifiers.length; i++) {
        var property = listOfModifiers[i];
        cssDefaultStyles += property + ':' + getInputCSS(property) + ';';
    }
    // End of CSS variable checks

    var text = input.value,
        textLen = text.length,
        fakeClone = document.createElement("div");
    if (selectionStart > 0) appendPart(0, selectionStart);
    var fakeRange = appendPart(selectionStart, selectionEnd);
    if (textLen > selectionEnd) appendPart(selectionEnd, textLen);

    // Styles to inherit the font styles of the element
    fakeClone.style.cssText = cssDefaultStyles;

    // Styles to position the text node at the desired position
    fakeClone.style.position = "absolute";
    fakeClone.style.top = topPos + "px";
    fakeClone.style.left = leftPos + "px";
    fakeClone.style.width = width + "px";
    fakeClone.style.height = height + "px";
    document.body.appendChild(fakeClone);
    var returnValue = fakeRange.getBoundingClientRect(); //Get rect

    if (!debug) fakeClone.parentNode.removeChild(fakeClone); //Remove temp
    return returnValue;

    // Local functions for readability of the previous code
    function appendPart(start, end) {
        var span = document.createElement("span");
        span.style.cssText = cssDefaultStyles; //Force styles to prevent unexpected results
        span.textContent = text.substring(start, end);
        fakeClone.appendChild(span);
        return span;
    }
    // Computing offset position
    function getInputOffset() {
        var body = document.body,
            win = document.defaultView,
            docElem = document.documentElement,
            box = document.createElement('div');
        box.style.paddingLeft = box.style.width = "1px";
        body.appendChild(box);
        var isBoxModel = box.offsetWidth == 2;
        body.removeChild(box);
        box = input.getBoundingClientRect();
        var clientTop = docElem.clientTop || body.clientTop || 0,
            clientLeft = docElem.clientLeft || body.clientLeft || 0,
            scrollTop = win.pageYOffset || isBoxModel && docElem.scrollTop || body.scrollTop,
            scrollLeft = win.pageXOffset || isBoxModel && docElem.scrollLeft || body.scrollLeft;
        return {
            top: box.top + scrollTop - clientTop,
            left: box.left + scrollLeft - clientLeft
        };
    }
    function getInputCSS(prop, isnumber) {
        var val = document.defaultView.getComputedStyle(input, null).getPropertyValue(prop);
        return isnumber ? parseFloat(val) : val;
    }
}