After "do JavaScript" my AppleScript variable becomes undefined

Question

I have declared global variables at the top of the script, and I have initialised them.

I can set the do JavaScript to the variable emptyDue fine. But then as soon as I try to compare the result of the JavaScript to the serviceDate variable, I get

error "The variable emptyDue is not defined." number -2753 from "emptyDue"

Why can I assign emptyDue on one line, and then on the very next line, it suddenly forgets that it should exist?

global emptyDue, serviceDate
set emptyDue to ""
set serviceDate to "2021-11-30"

on isEmptyDue()
    
    tell application "Safari" to tell document 1
        
        set emptyDue to do JavaScript "document.getElementById('trgUseNext').click();
 document.getElementById('trgUseNext').value; --gets the date value from a webpage."
        
        if emptyDue < serviceDate then
            display dialog "TRUE"
        else
            display dialog "FALSE"
        end if
        
    end tell
    
end isEmptyDue

isEmptyDue()
log emptyDue

So what do you think happens when that document element doesn't exist? — red_menace
– red_menace, Commented Nov 4, 2021 at 16:44
codeblog.jonskeet.uk/2010/08/29/writing-the-perfect-question "There’s no need to include greetings and sign-offs such as “Hi everyone!” and “Thanks – hope to get an answer soon” in the question. These will often be edited out by other users, as they’re basically a distraction. Greetings at the start of a question are particularly useless as they can take up valuable space in the snippet displayed in the question list." — epascarello
– epascarello, Commented Nov 4, 2021 at 16:44
@red_menace I don't understand the question, sorry. Which document element are you referring to? — improbablyconfused
– improbablyconfused, Commented Nov 4, 2021 at 16:46
@epascarello Hi there! Thanks for your comment, very much appreciated. It really helped me with my problem. Have a great day! — improbablyconfused
– improbablyconfused, Commented Nov 4, 2021 at 16:47
That would be the document.getElementById('trgUseNext') you are trying to use. — red_menace
– red_menace, Commented Nov 4, 2021 at 16:48

foo · Accepted Answer · 2021-11-06 07:37:12Z

2

There is a known flaw in AppleScript’s design: it is possible for a command to return no value. If you then bind the result of that command to a variable, it effectively makes that variable undefined.

You can avoid that -2753 error by making sure your command always returns a value, even if that value is just an empty string.

answered Nov 6, 2021 at 7:37

foo

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1 Comment

jsm Over a year ago

Great observation. Just to help others who are having the same problem: a very easy way to ensure that you always return a value, is to wrap your js command in a try block.

jsm · Accepted Answer · 2023-06-08 19:31:21Z

1

This is a known AppleScript bug, as pointed out by user foo's answer to this question. Here is a little code snippet that might help others work around the same problem:

on tryJavaScript(js)
    set outVal to ""
    set tried to 0
    set js to "try{" & js & "}catch{null};"
    
    tell application "Safari"
        
        tell current tab of first window to repeat
            set outVal to do JavaScript js
            try
                set working_around_error_2753 to outVal
            on error number -2753
                set outVal to "not found"
            end try

            if outVal is not "null" then exit repeat
            delay 0.1
            set tried to tried + 1
            if (tried > 5) then return "unrecoverable"
        end repeat
        
    end tell
    return outVal
end tryJavaScript

You would call this handler with the following syntax:

set emptyDue to tryJavaScript("document.getElementById('trgUseNext').click();
 document.getElementById('trgUseNext').value;")

Or, if you want an example that will work on the HTML of a StackOverflow page, try:

set myOutput to tryJavaScript("document.getElementById('footer').innerText;")

This won't guarantee that your JavaScript will work, but at least it will give you a more meaningful error message to work with.

edited Jun 8, 2023 at 19:31

answered Feb 13, 2023 at 15:44

jsm

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2 Comments

d-b Over a year ago

The line if outVal is not "null" then exit repeat can't be right? In case you get a -2753, outVal is set to "not found" and then it is not null and the loop exits, without trying 5 times.

jsm Over a year ago

It is right, because we need to allow for AppleScript not hearing back from the JavaScript in time. We want to exit the loop if we get either a valid result, or an error. If the result is "null", we may simply not have waited long enough. You could avoid the need for a loop by simply having a much longer timer, but this method exits as soon as we can.

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