3个点用法

function demo(...arr) { 
    arr.forEach(function (index,key) { 
        console.log(index) 
    });
}
demo(1, 2, 3, 4, 5);

function demo2(a,b,c) { 
    console.log(a)
    console.log(b)
    console.log(c)
}
let arr = [7, 8, 9];
demo2(...arr);