javascript "Cannot read property ' checked' of null"

Question

function piliang() {
    var ids = "";
    var num = document.getElementsByName("check");
    for (var i = 0; 1 < num.length; i++) {
        if (num.item(i).checked) {
            ids = ids + num.item(i).value + ",";
        }
        alert(ids);
    }

运行成功

function piliang() {
    var ids = "";
    var num = document.getElementsByName("check");
    for (var i = 0; 1 < num.length; i++) {
        if (num.item(i).checked) {
            ids = ids + num.item(i).value + ",";
        }
    }

这给出了以下错误:

"Cannot read property 'checked' of null"

第一段代码也有这个错误，但是可以运行成功

我的英文不好，请尽量用代码解释一下，非常感谢

Answer 1

修复了 for 循环

function piliang() {
    var ids = "";
    var num = document.getElementsByName("check");
    for (var i = 0; 1 < num.length; i++) {    //use i instead of 1
        if (num.item(i).checked) {
            ids = ids + num.item(i).value + ",";
        }
    }