javascript - 为什么我的未初始化变量可以工作？

Question

我正在制作一个具有两个阶段的计时器，但我忘记初始化我提供给计时器对象的函数中的一些变量。但它似乎工作起来没有任何问题。

例如，对于timerObj.startTime，我知道我将其添加到对象中并在那里定义它，然后在timerObj.start()内定义它。然而，我没有以timerObj为前缀的currentTime、minesLeft和secondLeft仍然可以完全正常工作，而无需全局定义或在间隔函数内定义。它们的处理方式与timerObj.startTime 相同吗？

var timerObj = {workDuration : 60000, breakDuration : 60000, currentPhase : "work"};

document.addEventListener("DOMContentLoaded", function(){
    document.getElementsByTagName("button")[4].addEventListener("click", timerObj.start);
    document.getElementsByTagName("button")[5].addEventListener("click", timerObj.stop);
    document.getElementsByTagName("button")[6].addEventListener("click", timerObj.pause);
});

timerObj.changeTime = function(durationType, durationChange) {
    return timerObj[durationType] = timerObj[durationType] + durationChange;
};

timerObj.start = function() {
    timerObj.startTime = new Date().getTime();
    if (timerObj.timeLeftOnPause === undefined){
        timerObj.timeLeftOnPause = timerObj[timerObj.currentPhase + "Duration"];
    }
    timerInterval = setInterval(timerObj.interval, 100);
};

timerObj.interval = function() {
    currentTime = new Date().getTime() - timerObj.startTime;
    timerObj.timeLeft = timerObj.timeLeftOnPause - currentTime;
    minutesLeft = numberDecorator(Math.floor(Math.ceil(timerObj.timeLeft / 100) / 600));
    secondsLeft = numberDecorator(Math.floor((Math.ceil(timerObj.timeLeft / 100) / 10) % 60));
    displayTime = minutesLeft + " : " + secondsLeft;
    document.getElementsByClassName(timerObj.currentPhase + "-timer-display")[0].innerHTML = displayTime;
    if (currentTime >= timerObj.timeLeftOnPause) {
        timerObj.stop();
        timerObj.currentPhase === "work" ? timerObj.currentPhase = "break" : timerObj.currentPhase = "work"; 
        if (timerObj.currentPhase === "break"){
            timerObj.start();
        }
    }
    return timerObj.timeLeft;
};

timerObj.pause = function(){
    timerObj.timeLeftOnPause = timerObj.timeLeft;
    clearInterval(timerInterval);
}

timerObj.stop = function() {
    clearInterval(timerInterval);
    timerObj.timeLeftOnPause = timerObj.workDuration;
    document.getElementsByClassName(timerObj.currentPhase + "-timer-display")[0].innerHTML = 
    numberDecorator(timerObj[timerObj.currentPhase + "Duration"] / 60000) + " : 00";
};

Answer 1

因为如果您将任何内容分配给不存在的变量，该变量将在全局范围内自动声明...除非您使用 use strict。