Javascript var hoisting issue

问题:

I have a very simple Javascript function that hits a MS SQL server and returns some records back. There is a cell that i want to only display in the top table row when it is unique. I believe my problem is var hoisting because the variable that i assign within the while loop does not work because the value is not carried from the last records to compare. Here is the code

function searchIndex()
{

    var termcounter = "";

    flyoutHTML = '<table>';

    var adOpenDynamic = 2
    var adLockOptimistic = 3

    var conn = new ActiveXObject("ADODB.Connection");
    var connectionstring = "Provider=SQLOLEDB;Server=XXXXXXXX;INTEGRATED SECURITY=SSPI;DATABASE=YYYYYYYYYY;"

    conn.Open(connectionstring)
    var rs = new ActiveXObject("ADODB.Recordset")

    rs.Open("SELECT field1, field2, field4, field4, field5 FROM dbo.table;", conn)

    if (rs.eof)
    {
    flyoutHTML += '<tr><td align="center">No Records Found!</td></tr>';
    }
    else
    {


    while(!rs.eof)
    {

    if (termcounter != rs(0))
    {
        var termcounter = rs(0);
        flyoutHTML += '<tr>';
        flyoutHTML += '<td colspan="3">' + rs(0) + '</td>';
        flyoutHTML += '</tr>';
    }


    flyoutHTML += '<tr>';
    flyoutHTML += '<td>' + rs(1) + '</td><td>' + rs(2) + '</td><td>' + rs(3) + '</td>';
    flyoutHTML += '</tr>';

    rs.movenext

    }
    rs.close
    conn.close


    flyoutHTML += '</table>';

}

回答1:

Take the "var" off the var termcounter = rs(0);

You're probably right about hoisting--JavaScript doesn't have block scope, so both times you declare termcounter it's in the same scope. This is a very common mistake, since JavaScript looks like C-based languages which have block scope.

Declaring a var in a conditional or looping block is equivalent to declaring it at the beginning of the function that block is in, scope-wise.

You'll lose less hair if you always place your var declarations at the top of functions--never in conditional or loop blocks. See Crockford. You can use JSLint to warn you. (Oh, by the way, you're missing a ton of semicolons--JSLint will hurt your feelings.)

So why are you redeclaring it? Drop the second "var" and just do the assignment.

---

The Microsoft stuff is alien to me, but I find the rs(0) syntax baffling. Is rs an object? Or is there some kind of magic that makes it into a function as well (you never know with JS)? I did some Googling and I wonder if you need to be using rs.fields(0) or rs.fields(0).name or rs.fields(0).value or something.

回答2:

You have this:

var termcounter = rs(0);

I don't think you want to be redeclaring it here - remove the var:

termcounter = rs(0);

回答3:

AKA How can one de-reference JavaScript variables when enclosing an outer scope?