通过函数附加全局数组显示为长度为零别处 [英] Global array appended by a function shows up as zero length elsewhere

问题描述

我已经声明数组在我的脚本的顶部标签了。

I have declared the array labels up at the top of my script

var labels = [];

有一个功能，retrieveLabels被称为追加到该数组：

A function, retrieveLabels is called to append to this array:

     function retrieveLabels() {
      labels = [];
      var getLabelsQuery = "SELECT DISTINCT label FROM items ORDER BY label;"
      db.transaction(function(tx) {
           tx.executeSql(getLabelsQuery, [],
                function(tx, labelsResults) {
                     for (var x = 0; x < labelsResults.rows.length; x++) {
                          var labelsRow = labelsResults.rows.item(x);
                          labels.push(labelsRow['label']);
                     }
                     console.log(labels.length);
                     console.log(labels);
                }
           );
      });
 }

第一个控制台消息显示34项。
第二个控制台消息显示[LABEL1，LABEL2.....label34]

The first console message shows 34 items. The second console message shows ["label1", "label2"....."label34"]

我在哪里调用该函数是在这里：

Where I call the function is here:

else {
 init_db();
 retrieveLabels();
 console.log(labels.length);
 console.log(labels);

}

第一个控制台消息显示0项。
第二个控制台消息显示[LABEL1，LABEL2.....label34]

The first console message shows 0 items. The second console message shows ["label1", "label2"....."label34"]

为什么会这样突然成了0长度的数组？或者是它同时修改了retrieveLabels内？

Why would this all of a sudden become a 0 length array? Or was it modified while inside of retrieveLabels?

推荐答案

我要说的是， tx.executeSql（）是异步的，并通过了匿名函数的参数3是以后实际执行的 retrieveLabels（）;返回

I would say that tx.executeSql() is asynchronous, and the anonymous functions passed as argument3 is actually executed after retrieveLabels(); has returned.

所以当时你的第二个的console.log（labels.length）; 被调用，查询可能不会被执行

So at the time your second console.log(labels.length); is called, the query may not have been executed.

下面是需要一个回调作为参数，并调用它一旦标签数组已被填充你的函数略加修改的版本：

Here is a slightly modified version of your function that takes a callback as argument, and calls it once the labels array has been populated:

function retrieveLabels(callback) {
    var labels = [];
    var getLabelsQuery = "SELECT DISTINCT label FROM items ORDER BY label;"
    db.transaction(function(tx) {
        tx.executeSql(getLabelsQuery, [],
            function(tx, labelsResults) {
                for (var x = 0; x < labelsResults.rows.length; x++) {
                    var labelsRow = labelsResults.rows.item(x);
                    labels.push(labelsRow['label']);
                }               

                callback(labels);
            }           
        );      
    }); 
}

然后就可以调用该函数是这样的：

Then you can call the function like this:

init_db();
retrieveLabels(function(label) {
    console.log(labels.length);
    console.log(labels);
});

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