检查数组是否为空或存在

问题描述

第一次加载页面时，我需要检查image_array中是否有图片并加载最后一张图片.

When the page is loading for the first time, I need to check if there is an image in image_array and load the last image.

否则，我会禁用预览按钮，提醒用户按下新图像按钮并创建一个空数组来放置图像；

Otherwise, I disable the preview buttons, alert the user to push new image button and create an empty array to put the images;

问题是 else 中的 image_array 一直触发.如果数组存在 - 它只是覆盖它，但警报不起作用.

The problem is that image_array in the else fires all time. If an array exists - it just overrides it, but alert doesn't work.

if(image_array.length > 0)
    $('#images').append('<img src="'+image_array[image_array.length-1]+'" class="images" id="1" />');
else{
    $('#prev_image').attr('disabled', 'true');
    $('#next_image').attr('disabled', 'true');
    alert('Please get new image');
    var image_array = [];
}

更新在加载 html 之前，我有这样的事情:

UPDATEBefore loading html, I have something like this:

<?php if(count($images) != 0): ?>
<script type="text/javascript">
    <?php echo "image_array = ".json_encode($images);?>
</script>
<?php endif; ?>

推荐答案

if (typeof image_array !== 'undefined' && image_array.length > 0) {
    // the array is defined and has at least one element
}

您的问题可能是由于隐式全局变量和变量提升的混合而发生的.确保在声明变量时使用 var:

Your problem may be happening due to a mix of implicit global variables and variable hoisting. Make sure you use var whenever declaring a variable:

<?php echo "var image_array = ".json_encode($images);?>
// add var  ^^^ here

然后确保您以后不会意外地重新声明该变量:

And then make sure you never accidently redeclare that variable later:

else {
    ...
    image_array = []; // no var here
}

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