Node.js Async.js 结合 node-mysql 使用的疑问

This topic created in 3799 days ago, the information mentioned may be changed or developed.

使用 Async.js 查询三条 SQL 语句并返回结果：

var sqls = { table_a: "select count(*) from table_a", table_b: "select count(*) from table_b", table_c: "select count(*) from table_c" }; async.map(sqls, function(item, callback) { connection.query(item, function(err, results) { callback(err, results); }); }, function(err, results) { if(err) { console.log(err); } else { console.log(results); } });

可以输出：
{ table_a: [ { 'count(*)': 26 } ], table_b: [ { 'count(*)': 3 } ], table_c: [ { 'count(*)': 2 ] }

根据 map 函数的文档：

arr - An array to iterate over.
iterator(item, callback) - A function to apply to each item in arr. The iterator is passed a callback(err, transformed) which must be called once it has completed with an error (which can be null) and a transformed item.
callback(err, results) - Optional A callback which is called when all iterator functions have finished, or an error occurs. Results is an array of the transformed items from the arr.

connection.query 函数符合第二个参数 iterator(item, callback)。

为什么以下的使用方式会报错：

var sqls = { table_a: "select count(*) from table_a", table_b: "select count(*) from table_b", table_c: "select count(*) from table_c" }; async.map(sqls, connection.query, function(err, results) { if(err) { console.log(err); } else { console.log(results); } });

TypeError: Cannot read property 'typeCast' of undefined

6 replies 2015-12-23 13:08:47 +08:00

allenfantasy

Dec 23, 2015

`arr - An array to iterate over`

但是你的 sqls 变量不是一个 array 啊

wucao219101

Dec 23, 2015

@allenfantasy map 好像是支持 Object 的，虽然文档上写的是 array 。

ablula

Dec 23, 2015

@wucao219101 改成：
```
async.map(sqls, connection.query.bind(connection), function(err, results) {
if(err) {
console.log(err);
} else {
console.log(results);
}
});
```

wucao219101

Dec 23, 2015

@sweetvvck 的确可以，大神能不能解释一下？

EPr2hh6LADQWqRVH

Dec 23, 2015

this

ablula

Dec 23, 2015

@wucao219101 直接将方法(connection.query)当参数传入另一个方法(async.map)默认会改变作为参数的方法的上下文(就是 @avastms 说的 this)，所以需要 bind 一下正确的上下文;