How to select line of text in textarea

This function expects first parameter to be reference to your textarea and second parameter to be the line number

function selectTextareaLine(tarea,lineNum) {
    lineNum--; // array starts at 0
    var lines = tarea.value.split("\n");

    // calculate start/end
    var startPos = 0, endPos = tarea.value.length;
    for(var x = 0; x < lines.length; x++) {
        if(x == lineNum) {
            break;
        }
        startPos += (lines[x].length+1);

    }

    var endPos = lines[lineNum].length+startPos;

    // do selection
    // Chrome / Firefox

    if(typeof(tarea.selectionStart) != "undefined") {
        tarea.focus();
        tarea.selectionStart = startPos;
        tarea.selectionEnd = endPos;
        return true;
    }

    // IE
    if (document.selection && document.selection.createRange) {
        tarea.focus();
        tarea.select();
        var range = document.selection.createRange();
        range.collapse(true);
        range.moveEnd("character", endPos);
        range.moveStart("character", startPos);
        range.select();
        return true;
    }

    return false;
}

Usage:

 var tarea = document.getElementById('myTextarea');
 selectTextareaLine(tarea,3); // selects line 3

Working example:

http://jsfiddle.net/5enfp/

The code in the post by darkheir does not work correctly. Based on it I shortened the code and made it working.

    function onClickSelection(textarea){
        if(typeof textarea.selectionStart=='undefined'){
            return false;
        }
        var startPos=(textarea.value.substring(0,textarea.selectionStart).lastIndexOf("\n")>=0)?textarea.value.substring(0,textarea.selectionStart).lastIndexOf("\n"):0;
        var endPos=(textarea.value.substring(textarea.selectionEnd,textarea.value.length).indexOf("\n")>=0)?textarea.selectionEnd+textarea.value.substring(textarea.selectionEnd,textarea.value.length).indexOf("\n"):textarea.value.length;
        textarea.selectionStart=startPos;
        textarea.selectionEnd=endPos;
        return true; 
    }

A somewhat neater version of the search for lines:

    function select_textarea_line (ta, line_index) {

        const newlines = [-1];   // Index of imaginary \n before first line
        for (let i = 0; i < ta.value.length; ++i) {
            if (ta.value[i] == '\n') newlines.push( i );
        }

        ta.focus();
        ta.selectionStart = newlines[line_index] + 1;
        ta.selectionEnd   = newlines[line_index + 1];

    } // select_textarea_line