[英]Javascript For Loop index returning “[object Object] - undefined”
我正在尝试将一个函数运行到许多元素,所以我正在使用for循环。 我不明白为什么我没有得到任何价值观。
var i;
var aFields = ["#business1A","#business1B","#business1C","#business1D","#business2A","#business2B","#business2C","#business2D",
"#business3A","#business3B","#business3C","#business3D","#business4A","#business4B","#business4C","#business4D",
"#business5A","#business5B","#business5C","#business5D","#business6A","#business6B","#business6C","#business6D"];
for (i = 0; i < aFields.length; i++) {
$(aFields[i]).keyup(function(){
alert($(aFields[i]+'Warning') + " - " +$(aFields[i]).val());
});
}
那是因为你在事件处理程序的回调函数中使用变量i
。 事件在循环结束时发生,因此变量包含的索引超出了数组中的最后一项。
要使用绑定事件的迭代中的变量值,可以使用立即执行的函数表达式通过创建范围为每次迭代创建变量:
var i;
var aFields = ["#business1A","#business1B","#business1C","#business1D","#business2A","#business2B","#business2C","#business2D",
"#business3A","#business3B","#business3C","#business3D","#business4A","#business4B","#business4C","#business4D",
"#business5A","#business5B","#business5C","#business5D","#business6A","#business6B","#business6C","#business6D"];
for (i = 0; i < aFields.length; i++) {
(function(j){
$(aFields[j]).keyup(function(){
alert($(aFields[j]+'Warning') + " - " +$(aFields[j]).val());
});
})(i);
}
尝试这个:
var i;
var aFields = ["#business1A","#business1B","#business1C","#business1D","#business2A","#business2B","#business2C","#business2D",
"#business3A","#business3B","#business3C","#business3D","#business4A","#business4B","#business4C","#business4D",
"#business5A","#business5B","#business5C","#business5D","#business6A","#business6B","#business6C","#business6D"];
for (i = 0; i < aFields.length; i++) {
(function(j){
$(aFields[j]).keyup(function(){
alert(($(this).attr('id')+'Warning') + " - " +$(this).val());
});
})(i);
}
这里的问题是闭包变量 i
的错误用法 ,变量在所有键盘处理程序之间共享,所以当循环结束时, i
将得到值aFields.length
因此aFields[i]
将返回undefined。
尝试
var aFields = ["#business1A", "#business1B", "#business1C", "#business1D", "#business2A", "#business2B", "#business2C", "#business2D",
"#business3A", "#business3B", "#business3C", "#business3D", "#business4A", "#business4B", "#business4C", "#business4D",
"#business5A", "#business5B", "#business5C", "#business5D", "#business6A", "#business6B", "#business6C", "#business6D"];
$.each(aFields, function (i, val) {
$(val).keyup(function () {
alert($(val + 'Warning') + " - " + $(val).val());
});
})
其他答案是关于闭包如何工作的很好的解释,但最干净的解决方案是使用this
来引用所选对象:
var i;
var aFields = ["#business1A","#business1B","#business1C","#business1D","#business2A","#business2B","#business2C","#business2D",
"#business3A","#business3B","#business3C","#business3D","#business4A","#business4B","#business4C","#business4D",
"#business5A","#business5B","#business5C","#business5D","#business6A","#business6B","#business6C","#business6D"];
for (i = 0; i < aFields.length; i++) {
$(aFields[i]).keyup(function(){
alert(this.id + ' Warning - ' + this.value);
});
}
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