为什么此“未捕获的ReferenceError:速度未定义"?错误? [英] Why this "Uncaught ReferenceError: tempo is not defined" error?

问题描述

我有以下代码:

$("#avvia_cronometro").click(function() {
    var tempo = setInterval(function() {
        cronometro();
        $("#tempo_cronometro").html((h)+":"+(min)+":"+(sec));
    }, 1000);
});
$("#stop_cronometro").click(function() {
    clearInterval(tempo);
});

function cronometro() {
    if (sec == 59) {
        min +=1;
        if (min == 59) {
            h+=1;
            min=0;
        }
        sec=0;
    }
    sec+=1;
}

当我单击#stop_cronometro时，它不起作用，并显示:

When I click on #stop_cronometro it doesn't work and it says:

Uncaught ReferenceError: tempo is not defined

我该如何解决?

如果我单击#avvia_cronometro，它会从时间开始，因此可以正常工作.

If I click on #avvia_cronometro it starts with the time so it's work.

推荐答案

因为全局范围(或​​停止点击处理程序可以到达的任何范围)中不存在变量tempo.

Because there is no variable tempo that exist in global scope (or any scope that the stop click handler can reach).

当您在函数内部使用var声明变量时，该变量将在该函数返回时被删除:

When you declare a variable with var inside a function that variable gets deleted when the function returns:

function foo () {
    var bar = 1;
}
foo();
console.log(bar); // uncaught reference error - "bar" doesn't exist

如果需要全局变量，请在不使用var的情况下使用它:

If you need a global variable, use it without the var:

function foo () {
    bar = 1;
}
foo();
console.log(bar); // prints 1

但是，我通常不建议这样做，因为这对于将来的维护者来说似乎是一个错误.而是在全局范围内显式声明全局变量，以清楚地表明您的意图:

However, I would generally not recommend this since it looks like an error to future maintainers. Instead, declare the global variable explicitly in global scope to clearly show your intention:

var bar = null;
function foo() {
    bar = 1;
}
foo();
console.log(bar); // prints 1

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