You can compare the document you are working on with another open document or with a file, and see the two texts side by side with their differences marked.
In the Textmode app 
on your Mac, bring the document you want to compare to the front.
Choose File > Compare With, then do one of the following:
Choose the name of another open document.
Choose “Choose File…”, then select a file.
A new window opens with the front document on the left and the other document or file on the right. The window is read-only: you can select and copy text in it, but you can’t edit it.
The two texts are lined up row by row, each side with its own line numbers, and scrolling one side scrolls the other.
A line that exists only on the left has a red tint, and an empty shaded row faces it on the right.
A line that exists only on the right has a green tint, and an empty shaded row faces it on the left.
A line that differs between the two sides is tinted red on the left and green on the right, and the characters that differ are marked more strongly.
The summary line above the texts counts the changes and the lines added and removed, for example “3 changes, +3 lines, -2 lines”. When there is no difference, it reads “The documents are identical.”
Line endings are not compared: the same text with LF, CRLF, or CR line endings is treated as equal, and a line ending after the last line is not a difference.
To go to the next change, choose View > Next Change, press Control-Option-↓, or click Next Change in the window.
To go to the previous change, choose View > Previous Change, press Control-Option-↑, or click Previous Change in the window.
The changed lines are selected on both sides. After the last change, Next Change returns to the first one.
The window shows the two texts as they were when you opened it. After you edit either document, click Refresh in the window to compare them again. Open documents are compared as they are now, including changes you haven’t saved. A file you chose is read again from disk.
Each side can have up to 200,000 lines. With a longer document, Textmode shows the message “These documents are too long to compare in this version.” and doesn’t open the window.
Compare With shows text documents. If you choose a file that doesn’t look like a text file, such as an image, Textmode shows a message that names the file and ends with “does not look like a text file.”, and doesn’t open the window.