为什么我会在 alert 调用 undef 中出现？
#!/usr/bin/env perl
use warnings;
use 5.014;
use utf8;
use Mojolicious::Lite;
use DBI;
my $db = 'my_test_db.db';
my $table = 'my_test_table';
my $dbh = DBI->connect( "dbi:SQLite:dbname=$db", '', '',
{ RaiseError => 1, PrintError => 0, AutoCommit => 1, sqlite_unicode => 1, }
) or die $DBI::errstr;
$dbh->do( "CREATE TEMP TABLE $table ( str TEXT, num INTEGER )" );
my $sth = $dbh->prepare( "INSERT INTO $table ( str, num ) VALUES ( ?, ?)" );
$sth->execute( 'aaa', '111' );
$sth->execute( 'bbb', '222' );
$sth->execute( 'ccc', '333' );
get '/eingabe' => sub {
my $self = shift;
$self->render( 'eingabe' );
};
get '/search_db/:col' => sub {
my $self = shift;
my $col = $self->param( 'col' );
my $term = $self->param( 'term' );
my $sth = $dbh->prepare( "SELECT DISTINCT $col FROM $table WHERE $col LIKE ?" );
$sth->execute( $term . '%');
my $ref;
while ( my $row = $sth->fetchrow_arrayref() ) {
push @$ref, @$row;
}
$self->render( json => $ref );
};
app->start;
__DATA__
@@ eingabe.html.ep
<!DOCTYPE HTML>
<html>
<head>
<meta charset="UTF-8" />
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.6.3/jquery.min.js"></script>
<script src="https://ajax.googleapis.com/ajax/libs/jqueryui/1.8.16/jquery-ui.js"></script>
<script type="text/javascript">
$( document ).ready( function() {
var ids = [ 'str', 'num' ];
for ( var i = 0; i < ids.length; i++ ){
$( "#" + ids[i] ).autocomplete({
source: function( request, response ){
alert( ids[i] ); // <---
$.getJSON( '/search_db/' + ids[i], request, function( data_from_server ){
var suggestions = [];
var len = data_from_server.length;
for( var i = 0; i < len; i++ ){
suggestions.push( data_from_server[i] );
}
response( suggestions );
});
}
});
}
});
</script>
</head>
<body>
<form>
<table>
<tr><td>String:</td><td><input type="text" id="str" name="str"" /></td></tr>
<tr><td>Number:</td><td><input type="number" id="num" name="num" /></td></tr>
</table><br />
<input type="submit" value="OK"/>
</form>
</body>
</html>
最佳答案
听起来您遇到了此处解释的问题:Creating closures in loops: A common mistake
您在调用 alert 时收到 undefined 的原因是，当调用 autocomplete 函数时，您的 for 循环已完成执行，意味着循环变量 i 的值为 2，即 ids.length + 1。因此，ids[i] 与 ids[2] 相同，后者不存在，因为 ids 数组中只有 2 个元素。我试图想出一个简单的演示来说明这种行为，以帮助说明发生了什么:http://jsfiddle.net/ianoxley/KwXVs/1/ (您需要打开浏览器的控制台才能查看结果)。
如果您创建了一个额外的闭包，它应该有助于保留您的范围并消除 未定义(请参阅 http://jsfiddle.net/ianoxley/BVa5Q/ )。
如果您尝试将代码更改为类似的内容，希望它能解决问题:
$(document).ready(function () {
function initAutocomplete(element_id) {
$("#" + element_id).autocomplete({
source: function (request, response) {
alert(element_id); // <---
$.getJSON('/search_db/' + element_id, request, function (data_from_server) {
var suggestions = [];
var len = data_from_server.length;
for (var i = 0; i < len; i++) {
suggestions.push(data_from_server[i]);
}
response(suggestions);
});
}
});
}
var ids = ['str', 'num'];
for (var i = 0; i < ids.length; i++) {
var current_id = ids[i];
initAutocomplete(current_id);
}
});
希望这有帮助。
关于Javascript:变量范围问题，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/7388248/