1.您的dns_types是字符串，因此需要将其解析为JSON
1.必须将data和dns_types用作两个单独的变量

var dns_types = '{"A":"ip","AAAA":"ipv6","CNAME":"target","MX":"","TXT":"","SRV":"","NS":""}';
var dnsTypesJson = JSON.parse(dns_types);

var data = [
    {
        "target": "1.2.3.5",
        "ref": 56283838,
        "host": "www.computerbuilder.co.uk",
        "type": "CNAME"
    },
    {
        "target": "129.0.0.1",
        "ref": 56283838,
        "host": "www.computerbuilder.co.uk",
        "type": "CNAME"
    }
];

for(var i in data) {
    let subData = data[i];
    let type = dnsTypesJson[subData.type];
  
    alert(subData[type]);
}