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/**
 * Return an 
 * @param {DOMElement} el A dom element of a textarea or input text.
 * @return {Object} reference Object with 2 properties (start and end) with the identifier of the location of the cursor and selected text.
 **/
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
    };
}

document.getElementById("trigger").addEventListener("click", function(){
	var input = document.getElementById("texto");
  var inputContent = input.value.length;
	// You may want to focus the textbox in case it's not
  input.focus();
  var result = getInputSelection(input);
  var resultSpan = document.getElementById("result");
  
  if(result.start == result.end){
  	resultSpan.innerHTML = "No text selected. Position of cursor is " + result.start +" of " +inputContent;
  }else{
  	resultSpan.innerHTML = "You've selected text ("+result.start+" to "+ result.end +") from a total length of " + inputContent;
  }
  
}, false);

<p>
 Get the cursor position in a textarea or a text input or the selected text.
</p><br>
<textarea id="texto"></textarea><br>
<input type="button" id="trigger" value="Check"/><br>
<span id="result"></span>