function sum(num){
 if(num <= 1){
 return 1;
 }else{
 console.log(num, arguments.callee(num-1));
 return num * arguments.callee(num-1);
 }
}

console.log(sum(3));

function box(num1,num2){
 return num1+num2;
}

function sum(num1,num2){
 //this 表示全局作用域，瀏覽器環(huán)境下window，node環(huán)境global，[]表示傳遞的參數(shù)
 return box.apply(this,[num1,num2]);

 //或者下面寫法arguments可以當(dāng)數(shù)組傳遞
 //return box.apply(this,arguments);
}

console.log(sum(10,10)); //

function box(num1,num2){
 return num1+num2;
}

function sum2(num1,num2){
 return box.call(this,num1,num2);
}

console.log(sum(10,10)); //