javascriptのwithの使い方

function students() {
        this.name = 'Jerry';
        this.age  = 18;
        this.score = function(yuwen,shuxue,waiyu) {
            return yuwen + shuxue + waiyu;
        }
    }


    stu = new students();


    //     
    stu.name = 'Jack';
    stu.age = 28;
    console.log("name : " + stu.name + ", age: " + stu.age + ", score : " + stu.score(85,90,92));


    //  with ，           
    with(stu) {
        name = 'tom';
        age = 23;
        console.log("name : " + name + ", age: " + age + ", score : " + score(60,80,50));
    }

<form onclick="return validForm(this);">
   <input type="text" name="username" id="username"/>
   <input type="submit"/>
</form>

//     
    //      
    function validForm1(formObj) {
        if(formObj.username.value == '') {
            console.log('username can not null.');
            return false;
        }
    }


    //  with 
    function validForm(formObj) {
        with(formObj) {
            if(username.value == '') {
                console.log('username can not null.');
                return false;
            }
        }
    }