JavaScriptの関数の引数が想定していない値ならデフォルト値を強制する（つっこみ待ち） ref: http://qiita.com/webbingstudio@github/items/6e7a77ed2347a59efedd

file0.txt

var example = function( current, value ){
    changed = Number(current) + Number(value);
    // changedで何かする
}

$('a.js-example').click(function(){
    var current = docCookies.getItem('current');
    var value = $(this).data('value');
    example( current, value );
    return false;
});

function_default_value.js

var default_value = function( val, default_val, nan_check ){
    var return_val = val == undefined ? default_val : val;
    return_val = val === null ? default_val : val;
    if( nan_check === 1 ) {
        return_val = isNaN(val) ? default_val : val;
    }
    return return_val;
};

var example = function( cur, val ){
    var current = default_value( cur, 0, 1 );
    var value = default_value( val, 0, 1 );
    changed = Number(current) + Number(value);
    // changedで何かする
}

$('a.js-example').click(function(){
    var current = docCookies.getItem('current');
    var value = $(this).data('value');
    example( current, value );
    return false;
});