无法读取未定义错误的属性“长度”

Question

I have downloaded the following web-based-face-detect-program and compiled it and my compiler shows no syntax errors, however when I try upload a picture with the application I get a Cannot read property 'length' of undefined error . I haven't changed to application at all so there should not be an issue and I am fairly new to Javascript so I could be making a rookie mistake. Heres the js function code:

function parse_response(res) {
    var data = jQuery.parseJSON(res);

    if (data.success == false) {
        show_page(form);
        show_message(data.msg);
        return;
    }

    results
        .find(".source-img").html("").end()
        .find(".detected-faces").html("").append("<p>Found " + (data.images.length-1) + " faces:</p>");

    $.each(data.images, function(i, img) {
        var el = '<img src="face-detect/' + img.src + '?r=' + Math.random() + '" alt="" />';
        results.find((i == 0 ? ".source-img" : ".detected-faces")).append(el);
    })

    results.find(".source-img img").bind('click', function(e) {
        var im = data.images[0];
        var el = '<img src="face-detect/' + im.src + '" width="' + im.width + '" height="' + im.height + '" alt="" />';
        $(".lightbox", results).find(".lightbox-content").html(el).end().lightbox("show");
    });

    show_page(results, actions);
}

The line .find(".detected-faces").html("").append("<p>Found " + (data.images.length-1) + " faces:</p>"); is what causes the error. And heres the errors I get in the Chrome console:

Uncaught TypeError: Cannot read property 'length' of undefined app.js:87
parse_response app.js:87
xhr.onreadystatechange app.js:65

The rest of the code is all on the github link if theres something I have missed out. So can someone please explain to me why I am getting this undefined error and how I can resolve it?

Answer 1

It seems that the data object have not the images property. Before the error line, use the JSON.stringify(data) function to check the data object structure.

You can use:

console.log(JSON.stringify(data))

Or:

alert(JSON.stringify(data))