点击到另个按钮的时候怎么取消掉上个按钮的背景色

点击到另个按钮的时候怎么取消掉上个按钮的背景色呢？
点击到另个按钮的时候怎么取消掉上个按钮的背景色呢？

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定义个类.blue(background: blue, color:#fff), 点击的时候先根据blue类名，找出有这个类的删掉元素的这个类，然后再给当前点击对象添加blue类

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .province {
            background: #eee;
            color: #000;
            padding: 2px;
            border-radius: 4px;
            display: inline-block;
        }
        .blue {
            background: blue;
            color: #fff;
        }
    </style>
</head>
<body>
    <div class="all">
        <div class="province">北京</div>
        <div class="province">天津</div>
        <div class="province">上海</div>
    </div>
</body>
<script>
    let proList = document.getElementsByClassName('province')
    for (let i = 0; i < proList.length; i++) {
        proList[i].onclick = function () {
            let activeItem = document.getElementsByClassName('blue')
            if (activeItem.length > 0) {
                activeItem[0].classList.remove('blue')
            }
            proList[i].classList.add('blue')
        }
    }
</script>
</html>