数组null无法在JavaScript中进行验证

Question

In my javascript i am trying to check an array if empty.If there is no item in <li> then array will be empty and this should throw error but it is not working. Here is my code

 var phrases = [];
        $('#listDiv #hiddenItemList').each(function () {
            var phrase = '';
            $(this).find('li').each(function () {
                var current = $(this);
                phrase += $(this).text() + ";";
            });
            phrases.push(phrase);
        });

        if (phrases === undefined || phrases.length == 0 )
        {
            $.alert("Please select rate type, high rate and low rate", {
                title: "Rates Info",
                type: "danger"
            });
            return false;
        }

Answer 1

You have to check that you're not just pushing an empty string into the array. This will make the array phrases have length and not be undefined but won't be what you're looking for.

    var phrases = [];
            $('#listDiv #hiddenItemList').each(function () {
                var phrase = '';
                $(this).find('li').each(function () {
                    var current = $(this);
                    phrase += $(this).text() + ";";
                });
                if ( phrase != '' ) {
                  phrases.push(phrase);
                }
            });

            if (phrases === undefined || phrases.length == 0 )
            {
                $.alert("Please select rate type, high rate and low rate", {
                    title: "Rates Info",
                    type: "danger"
                });
                return false;
            }