Why do I keep getting undefined in Acrobat Javascript console?

Just to add to the correct replies made above: "undefined" is not an error. The opposite, actually. It just means the code finished executing correctly, without throwing any errors or returning any values.

The issues with your code cause it to not actually do anything, but it is running.

++ Adding to the discussion,

Would you mind describing what where you trying to achieve and why are you trying to declare a variable using "let"?

After much reading, I am almost convinced that you can't declare variables using "let" with  Acrobat JavaScript.

As illustrated by NesaNurani, when you script with an Acrobat PDF you must use "var" to declare variables. In addition, if you also write the same line of code and omit "var" when you express the for-loop function (shown in the example below), Acrobat JavaScript will automatically assume the variable declaration for the "i" indicator:

for( i = 0; i < 10; i++){
console.println(i);
};

In both instances (NesaNurani's correct example and the example illustrated above), the "i" indicator is initialized  as a global variable inside of the document that you're working with. Both examples will execute the same results.

Seems to me like you were testing this script to work around something in Acrobat like a server-side script to test with web browsers, for example (in which case is not directly supported in Acrobat).

See if these additional discussions are relevant:

• https://answers.acrobatusers.com/Need-server-side-script-data-exchange-PDF-forms-data-q293094.aspx
• https://answers.acrobatusers.com/Server-side-scripting-q17412.aspx
• https://stackoverflow.com/questions/762011/whats-the-difference-between-using-let-and-var
• https://www.tutorialspoint.com/difference-between-var-and-let-in-javascript

As for "console.log", which is used to write or display data to the browser console, the standard output in Acrobat JavaScript is expressed with "console.println" instead. Otherwise, if successful, the Acrobat program will produce the write operation of the data transfer silently.

Use 'var' instead of 'let' and use 'println' instead of 'log'.

for(var i = 0; i < 10; i++){
console.println(i);
};