数组中查找第一次出现的元素
有这样的需求：我们需要找到返回数据中第一次出现的元素，并在第一个元素之前加上标题，代码如下：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
</head>
<body>
<div id="demo"></div>
<script>
let data = [
{
title: "5454545454",
flag: "a"
},
{
title: "5454545454",
flag: "a"
},
{
title: "5454545454",
flag: "b"
},
{
title: "5454545454",
flag: "c"
},
{
title: "5454545454",
flag: "c"
}
];
let flagNum = data.findIndex(item => item.flag === 'c');
document.getElementById('demo').innerHTML = data.map((item, index) => {
let title = flagNum === index ? '<h1>我是c模块的标题</h1>' : '';
return `${title}<p>${item.title}</p>`;
}).join('');
</script>
</body>
</html>
参考： findeIndex
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