Question

我制作了一个像这样的JavaScript计数器：

window.onload = function(){
    var target_date = new Date("Aug, 15, 2019").getTime();
}
var days, hours, minutes, seconds;
var countdown = document.getElementById("countdown");

setInterval(function (){
    var current_date = new Date().getTime();
    var seconds_left = (target_date - current_date) / 1000;

    days = parseInt(seconds_left / 86400);
    seconds_left = seconds_left % 86400;

    hours = parseInt(seconds_left / 3600);
    seconds_left = seconds_left % 3600;

    minutes = parseInt(seconds_left / 60);
    seconds = parseInt(seconds_left % 60);

    countdown.innerHTML= days + "d, " + hours + "h, " + minutes + "m, " + seconds + "s"; 
}, 1000);

HTML：

<span id="countdown"></span>

浏览器（Google Chorme）说：

Uncaught ReferenceError: target_date is not defined

即使我删除window.onload = function(){}，仍然无效。我做错了什么？

Answer 1

这与variable scope有关。在target_date中定义了window.onload，使其成为 local 。如果要在该函数之外使用该变量，请将其全局声明为，方法是将其移出函数：

var target_date; /* Declared globally. */
window.onload = function(){
    target_date = new Date("Aug, 15, 2019").getTime();
}

Answer 2

将您的第一个"}"更改为脚本的末尾

window.onload = function(){
            var target_date = new Date("Aug, 15, 2019").getTime();

        var days, hours, minutes, seconds;
        var countdown = document.getElementById("countdown");

        setInterval(function (){
            var current_date = new Date().getTime();
            var seconds_left = (target_date - current_date) / 1000;

            days = parseInt(seconds_left / 86400);
            seconds_left = seconds_left % 86400;

            hours = parseInt(seconds_left / 3600);
            seconds_left = seconds_left % 3600;

            minutes = parseInt(seconds_left / 60);
            seconds = parseInt(seconds_left % 60);

            countdown.innerHTML= days + "d, " + hours + "h, " + minutes + "m, " + seconds + "s"; 
        }, 1000); 
        }

Answer 3

只需将window.onload的大括号"}"移动到脚本的末尾

即可

window.onload = function(){
    var target_date = new Date("Aug, 15, 2019").getTime();
    var days, hours, minutes, seconds;
    var countdown = document.getElementById("countdown");

    setInterval(function (){
        var current_date = new Date().getTime();
        var seconds_left = (target_date - current_date) / 1000;

        days = parseInt(seconds_left / 86400);
        seconds_left = seconds_left % 86400;
        hours = parseInt(seconds_left / 3600);
        seconds_left = seconds_left % 3600;

        minutes = parseInt(seconds_left / 60);
        seconds = parseInt(seconds_left % 60);

        countdown.innerHTML= days + "d, " + hours + "h, " + minutes + "m, " + seconds + "s"; 
    }, 1000);
}

JavaScript计数器不起作用

3 个答案: