applescript getInputByClass2 with Safari 10.1 - variables

Question

My Applescripts I used to run everyday to get text from Safari isn't working since my last system update
It used to work only in Safari, and not in Safari Preview, I guess the system for Safari Preview was bring to safari now/
tell application "Safari"
set DinfoGrab to do JavaScript "
document.getElementsByClassName(' field type-string field-Dinfo ')[0].innerHTML;" in tab 3 of window 1
end tell
with this error :
Safari got an error: Can’t make " document.getElementsByClassName('
field type-string field-Dinfo ')[0].innerHTML;" into type text.
how can I fix that? thanks.
UPDATE :
Here is something working perfectly with Chrome :
tell application "Google Chrome"
tell tab 3 of window 1 to set r to execute javascript "document.getElementsByClassName('field type-string field-Dinfo')[0].innerHTML;"
end tell

Answer 1

Without seeing the complete code, I can't say for sure what's going on. But judging by the name of your function -- getInputByClass2 -- I assume you're trying to get the value of HTML <input> fields. If this is true, you should be using outPut.push(arr[i].value) instead of outPut.push(arr[i].innerHTML)
As for the second bit of code, your JavaScript doesn't have any error handling in case the value of document.getElementsByClassName(' field type-string field-Dinfo ')[0] is null.
var els = document.getElementsByClassName(' field type-string field-Dinfo ');
//set to value of [0].innerHTML if [0] exists, else empty string
var html = els.length ? els[0].innerHTML : "";
//return value to AppleScript
html;
update (response to updated question)
Running the following script in Script Editor against this StackOverflow page will return the correct value (assuming you have the correct window/tab numbers set). If the search field at the top of this StackOverflow page is empty, you will get an empty string. If you enter a term (but don't submit) then run the AppleScript, you will get the value of the field.
tell application "Safari"
set DinfoGrab to do JavaScript "
document.getElementsByClassName('js-search-field')[0].value;" in tab 1 of window 1
end tell
The only changes from your script are the window/tab numbers, the classname (changed to match the StackOverflow page), and I used value instead of innerHTML.
I have tested in the most current version of Safari (10.0.3); if this doesn't work in your version of Safari, ensure you're pointing to the correct class name. If this script DOES work for you, then the issue is probably due to something on the page you're trying to search, perhaps related to the type of <input> field you're fetching or an incorrect classname. Maybe the update to Safari is causing the page to render differently, which indirectly affects your code.