js requestAnimationFrame固定每秒60帧数

根据提供的引用内容，requestAnimationFrame并不能固定每秒60帧数，它只是在浏览器重绘之前调用回调函数，以便进行动画渲染等操作。如果要固定每秒60帧数，可以在回调函数中使用时间戳来计算每一帧之间的时间间隔，从而控制动画的帧率。

以下是一个使用requestAnimationFrame实现固定每秒60帧数的例子：

let lastTime = 0;
let vendors = ['webkit', 'moz'];
for (let i = 0; i < vendors.length && !window.requestAnimationFrame; ++i) {
    window.requestAnimationFrame = window[vendors[i] + 'RequestAnimationFrame'];
    window.cancelAnimationFrame = window[vendors[i] + 'CancelAnimationFrame'] || window[vendors[i] + 'CancelRequestAnimationFrame'];
}

if (!window.requestAnimationFrame) {
    window.requestAnimationFrame = function(callback, element) {
        let currTime = new Date().getTime();
        let timeToCall = Math.max(0, 16 - (currTime - lastTime));
        let id = window.setTimeout(function() {
            callback(currTime + timeToCall);
        }, timeToCall);
        lastTime = currTime + timeToCall;
        return id;
    };
}

if (!window.cancelAnimationFrame) {
    window.cancelAnimationFrame = function(id) {
        clearTimeout(id);
    };
}

let fps = 60;
let interval = 1000 / fps;
let then = Date.now();
let startTime = then;
let endTime = startTime + 5000;

function draw() {
    let now = Date.now();
    let elapsed = now - then;

    if (now < endTime) {
        requestAnimationFrame(draw);
    }

    if (elapsed > interval) {
        then = now - (elapsed % interval);
        // 在这里进行动画渲染等操作
    }
}

requestAnimationFrame(draw);