Here is an example:

    var clientContext = SP.ClientContext.get_current();
    var list = clientContext.get_web().get_lists().getByTitle("Title");
    var item = list.getItemById(id);
    var fieldValuesAsText = item.get_fieldValuesAsText();

    clientContext.load(fieldValuesAsText);
    clientContext.executeQueryAsync(function (sender, args) {
        var fieldValues = fieldValuesAsText.get_fieldValues(); // Text values of all fields
        var fieldValue = fieldValuesAsText.get_item('Title'); // Text value of one field
    }, function (sender, args) {
        alert(args.get_message());
    });