Effective JavaScript Ch.1

https://www.slideshare.net/teppeis/effective-java-script-ch1?next_slideshow=1

未定義チェック

function do(num, str) {

    if (typeof num === 'undefined') {
        num = 10;
    }
    if (num == null {
        num = 10;
    }

    //...........
}