push()方法将元素添加进数组却无法提取，原因何在？

Question

需求：遍历一个文件夹下的html文件，然后生成页面标题等数组。
问题：push进各数组的元素在console里可以看到，且数组长度正常，但是调用的时候显示为空。

// Add window load listener
window.addEventListener('load', showIndex);

function showIndex() {
    
    const files = ['test', 'demo'];

    for(let a = 0; a < 1; a++){
        
        let partNames = [];
        let sameParts = [];
        let pageUrl = [];
        let pageNames = [];
        
        for(let i = 0; i < 4; i++){
            // Open file
            let indexs = new XMLHttpRequest();
            
            indexs.open('GET', `../indexTest/${files[a]}${i}.html`,true);
            
            indexs.onload = function() {
                
                // Get part names and times
                let partName = getPageNames(indexs, 'h2');
                
                if(! partNames.includes(partName)){
                    partNames.push(partName);
                    sameParts.push(1);
                } else {
                    sameParts[sameParts.length - 1]++;
                }

                // Get page relative path
                pageUrl.push(`${files[a]}${i}.html`);

                // Get page names
                pageNames.push(getPageNames(indexs, 'h3'));
                
            };
            
            indexs.send();
            
        }
        
        // test
        console.log(partNames, sameParts, pageUrl, pageNames, files);
        console.log(partNames[0]);
        console.log('partNames.length: ' + partNames.length);
        console.log(Object.getOwnPropertyNames(pageUrl));

    }
}

function getPageNames(obj, hx) {

    let start = obj.responseText.indexOf(`<${hx}>`) + 4;
    let end = obj.responseText.indexOf(`</${hx}>`);
    
    return obj.responseText.substring(start, end);
}

console结果如下：

Answer 1

for循环里是个异步操作吧，console写到indexs.onload里试试，我觉得用递归好点

const loop = (i) => {
    if(i< 5) {
        // 模拟异步方法
        setTimeout(() => {
            i++;
            loop(i);
        }, 500);
    }
}
loop(0);