Here are some common flags used in NodeJS:

• 'a': Open file for appending. The file is created if it does not exist.
• 'w': Open file for writing. The file is created (if it does not exist) or truncated (if it exists). This is the default flag.
• 'r': Open file for reading. An exception occurs if the file does not exist.
• 'ax': Like 'a' but fails if the path exists. The ‘x’ in the ‘ax’ flag stands for “exclusive”.
• 'wx': Like 'w' but fails if the path exists.

Here is an example of how to use these flags:

// Append to the file
fs.writeFileSync("test.json", "[\n", { flag: 'a' })

// Write to the file (default behavior)
fs.writeFileSync("test.json", JSON.stringify(data, null, 2))

// Open file for reading
fs.readFileSync("test.json", { flag: 'r' })