First of all, it depends on what you mean by "original source" code.

• If you mean the exact same file on the exact same file system that you were editing when you wrote the software, the answer is that you can't. Obviously.

• If you mean code which is character for character identical to the code you wrote, it is technically possible but unlikely in practice.

• If you mean code that works exactly the same and "looks mostly the same", then yes, you can. (Depending on "mostly".)

The answer also depends on what AST implementation you are talking about.

• Some AST implementations don't preserve comments or spacing / indentation.

• Other AST implementations apparently can preserve comments; e.g. as decorations on the tree nodes.

• It is theoretically possible for an AST implementation to preserve absolutely everything needed to reconstruct identical source code. (But I don't know of an example that does. It would be memory expensive and kind of pointless.)

What is the harm in not being able to recover comments?

Well it depends on what you want to use the regenerated source code for. If you intend to be able to replace the original code, then there are clear problems:

• You have lost any (hopefully) useful comments that the programmer included to help people to understand the code.

• It is common to embed formal API documentation in the form of stylized source code comments that are then extracted, formatted, etc. If those comments are lost, it becomes harder to keep the API documentation up to date.

• Some 3rd-party tools use stylized comments for specific purposes. For example, a comment could be could be used to suppress a false positive from a static code analyzer; e.g. a # noqa comment in Python code suppresses a pep8 style error.

On the other hand ... this kind of thing may not be relevant for your use-case.

Now from the tags I deduce that you are using Shift-AST. From a brief scan of the documentation and source code, I don't think this preserves either comments or indentation / spacing.

So that means that you cannot recover source code that is character for character identical with the original code. If that is what you want ... your colleague is 100% correct.

However, character for character identical code may not be necessary, so this may not be a limitation. It depends on your use-case.

And you could investigate Babel as an alternative. Apparently it can preserve comments.

One of colleague told me AST does not preserve the spacing and indentation will be lost while converting AST to source code. Is it the only reason?

Clearly, No. (As my answer explains.)

Nope. An abstract syntax tree is abstract in the way that it abstracts away ambiguous grammar, such as whitespace and possibly also comments (if these are irrelevant to further processing). As there is usually no purpose in storing this information, it is worth dropping during parsing.

While one can't go back to "original source code", one can still go back to an equivalent representation which is usually called the canonical form.

Well, it's possible. If you're using shift-ast you can do it.

Step 1:

npm install shift-codegen

Step 2:

import codegen from "shift-codegen";
let programSource = codegen(/* Shift format AST */);

ProgramSource will return string. Write it to your file and use prettier to format your code.

There is an alternative to shift-ast is called babel gives so many benefits, transforms and template features. Also provides typescript, flow, jsx and comments and minification features.