Question

I have an object as follows:

    var dataSources = [
        {
            Stage:  [2, 3, 4],
            Name:   "GetAustralianStateList"
        },
        {
            Stage: [2, 3, 4],
            Name: "GetGenderList"
        },
        {
            Stage: [2, 3, 4],
            Name: "GetTitleList"
        },
        {
            Stage: [2, 3, 4],
            Name: "GetCountryList"
        },
        {
            Stage: [2, 3, 4],
            Name: "GetRegionList"
        },
        {
            Stage: [2, 3, 4],
            Name: "GetNonEnglishLanguageList"
        },
        {
            Stage: [2, 3, 4],
            Name: "GetContactRelationshipList"
        },
        {
            Stage: [3, 4],
            Name: "GetCompanyCodeList"
        },
        {
            Stage: [3, 4],
            Name: "GetBusinessContractList"
        },
        {
            Stage: [3, 4],
            Name: "GetPayrollAreaList"
        },
        {
            Stage: [3, 4],
            Name: "GetAdministrationAreaList"
        },
        {
            Stage: [3, 4],
            Name: "GetWorkContractList"
        },
        {
            Stage: [2, 3, 4],
            Name: "GetFirstLanguageList"
        },
    ];

When I call dataSources.length in Chrome it returns the correct number of items in the object. However Internet Explorer returns a length of 14 – 1 more than is actually in the object.

Any ideas?

Answer 1

Unfortunately you’ve discovered one of the very few bugs in ECMAScript implementations: IE treats a single trailing comma in an array literal as an elision, so it increments the length by one:

var a = [0,1,];

alert(a.length);  // 3 in IE
                  // 2 in other browsers

a.hasOwnProperty('2'); // false in all browsers

The above shows that IE treats the comma as an elision: the length has been increased by one but there is no member of the array at index 2.