数组查询includes

 let arr1 = ['kk', 'jo', 'll'];
            if (arr1.includes("kk")) {//【ɪnˈkluːz】
                console.log("找到了", arr1.includes("kk"));//true
            } else {
                console.log("没有", arr1.includes("kk"));
            }
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            let arr2 = ['kk', 'jo', 'll'];
            if (arr2.includes("kk", 1)) {
               //第二个参数，表示从数组中下标为1包含1处开始找
                console.log("找到了", arr2.includes("kk"));
            } else {
                console.log("没有", arr2.includes("kk"));//false
            }

includes可以应用在上传文件类型是否正确。是非常好用的。