我有一个名为 front images 的数组，写成var frontImages = [];. 据我了解，javascript 数组不需要指定长度。当我运行下面的代码时，它会在输出到控制台时打印 0 作为数组的长度。但是，当我将数组长度指定为 时var frontImages = new Array(3);，它会打印出 3，即数组的正确长度。为什么未指定长度的数组不返回 3？
function getMainPosters() {
var games = new Image(); //create 3 images
var apps = new Image();
var aboutme = new Image();
games.onload = function() { //add image to frontImages array on load
frontImages.push(games);
};
apps.onload = function() {
frontImages.push(apps);
};
aboutme.onload = function() {
frontImages.push(aboutme);
};
games.src = "images/assets/posters/games_poster.jpg"; //specify source of image
apps.src = "images/assets/posters/apps_poster.jpg";
aboutme.src = "images/assets/posters/aboutme_poster.jpg";
console.log(frontImages.length); //print the length of frontImages to console
}