Crear vista previa de imagen al colocarla en un input type text

<div class="box-previsualizar">
	<img src="" class="img-previsualizar" style="display: none;">
</div>

<input type="text" class="input-url" placeholder="Ingresa la url de la imagen...">
<button type="button" class="btn-previsualizar">Previsualizar!</button>

<script>
$(document).ready(function(){
	
	$(document).on('click', '.btn-previsualizar', function(){
		
		var inputurl = $('.input-url').val();
		
		if(inputurl !== ''){
			$('.img-previsualizar').attr('src', inputurl).show();
		} else {
			alert('El input esta vacio!');
		}
		
	});
	
});
</script>

	echo'
	<script>
	$(document).ready(function(){
    
    	$(document).on(\'click\', \'.btn-succes\', function(){
        
        	var inputurl = $(\'#avatar\').val();
        
        	if(inputurl !== \'\'){
            	$(\'#miAvatar\').attr(\'src\', inputurl).show();
        	} else {
            	alert(\'El input esta vacio!\');
        	}
    	});
    });
	</script>  ';

	echo'<div class="panel panel-default">
		<div class="panel-heading">
			<h3 class="panel-title">Cambiar avatar</h3>
		</div>
		<div class="panel-body">
			<div class="row">
				<div class="col-sm-8">
					<form onsubmit="return load_new_avatar();"  action="/users/change_avatar" method="POST">
						<input type="hidden" name="user" value="'.$row['id_u'].'" />
						<div class="form-group">
							<label>Avatar:</label>
							<input type="text" name="avatar" id="avatar" onfocus="foco(this);" class="form-control" required />
							<label id="errorss"></label>
						</div>
						<button type="button" class="btn btn-succes" />Previsulizar</button>
						<button type="submit" class="btn btn-default">Actualizar avatar</button>
					</form>
				</div>
				<div class="col-sm-4">
					<img src="'.$row['avatar'].'" class="img-responsive img-circle" id="miAvatar" />
				</div>
			</div>
		</div>
	</div>';

$(document).on('change', '#avatar', function(){

    var inputurl = $('#avatar').val(); 
    if(inputurl !== ''){
        $('#miAvatar').attr('src', inputurl).show();
    } else {
        alert('El input esta vacio!');
    }

});