问JavaScript -全局变量不变

EN

<input type="file" id="input" multiple onchange="readFiles(this.files)">

theFileList = null;

function test() {
    if (theFileList)
        alert(theFileList[0]);
    else
        alert("It is null!");
}

function readFiles(fileList) {
    if (fileList) {
        // I generate a new page in the "main" frame:
        var doc = parent.document.getElementById('main').contentWindow.document;
        theFileList = fileList;
        test(); // works fine (displays "[object File]").

        // opens html and head tags; writes head section (with stylesheet and external js; closes head and opens body:
        generateHeader(); 
        // create a button to check if global variable is null:
        doc.writeln("<button onclick=\"javascript:test();\">Click</button>");
        // closes body and html tags:
        generateFooter();

        doc.close();
    }
    else
        alert("Something went wrong");
}