回調函數

函數是對象，也就意味着函數能夠看成參數傳入另一個函數中。給函數writeCode()傳入一個函數參數introduceBugs()，在某個時刻writeCode()執行了（或調用了）introduceBugs()，在這種狀況下，咱們稱introduceBugs()是一個「回調函數」，簡稱「回調」

 

碰撞事例

var lastTime = Date.now();
//速度
var box1Speed = 100;
var box2Speed = 150;

var box1 = document.getElementById("box1");
var box2 = document.getElementById("box2");
function collides(rectA,rectB){
return !( rectA.y + rectA.height < rectB.y ||
rectB.y + rectB.height < rectA.y ||
rectA.x + rectA.width < rectB.x ||
rectB.x + rectB.width < rectA.x
)
}
function widhtHeight(element){
return {
x: element.offsetLeft,
y: element.offsetHeight,

width: element.offsetWidth,
height: element.offsetHeight
}
}
function main(){
var now = Date.now();
var dt = (now - lastTime) / 1000;

var rectA = widhtHeight(box1);
var rectB = widhtHeight(box2);

var flag = collides(rectA,rectB);
if(flag){
alert("碰撞");
return;
}else{
box1.style.left = parseInt(box1.style.left) + box1Speed * dt + "px";
box2.style.left = parseInt(box2.style.left) - box2Speed * dt + "px";
}

lastTime = now;
setTimeout(main,1000/60);
}
main();