Fastmod: A Powerful Code Replacement Tool (with examples)

Fastmod is a command-line tool developed by Facebook that provides a fast and efficient way to partially replace code in a codebase. It is a great alternative to the codemod tool, offering better performance and a simpler syntax. In this article, we will explore different use cases of Fastmod, and explain each command along with their arguments and outputs.

Use Case 1: Replace a regex pattern in all files of the current directory

Code:

fastmod regex_pattern replacement

Motivation: This command can be useful when you want to replace a specific pattern with a new value in multiple files within the current directory.

Explanation:

• regex_pattern represents the regular expression pattern that should be matched.
• replacement represents the new value that should replace the matched pattern.

Example Output:

Initializing fastmod with 4 workers
Searching for regex_pattern in all files within the current directory...
Found 10 matches in 5 files.
Replacing regex_pattern with replacement...
Replaced 10 occurrences in 5 files.

Use Case 2: Replace a regex pattern in case-insensitive mode in specific files or directories

Code:

fastmod --ignore-case regex_pattern replacement -- path/to/file path/to/directory ...

Motivation: This command allows you to perform a case-insensitive search and replace operation in specific files or directories.

Explanation:

• --ignore-case flag is used to enable case-insensitive mode for the regex pattern matching.
• regex_pattern represents the regular expression pattern that should be matched.
• replacement represents the new value that should replace the matched pattern.
• -- path/to/file path/to/directory ... specifies the files or directories where the search and replace operation should be performed.

Example Output:

Initializing fastmod with 4 workers
Searching for regex_pattern in specified files or directories...
Found 5 matches in 2 files.
Replacing regex_pattern with replacement...
Replaced 5 occurrences in 2 files.

Use Case 3: Replace a regex pattern in a specific directory in files filtered with a case-insensitive glob pattern

Code:

fastmod regex replacement --dir path/to/directory --iglob '**/*.{js,json}'

Motivation: This command allows you to replace a regex pattern in a specific directory, considering only files that match a case-insensitive glob pattern.

Explanation:

• regex represents the regular expression pattern that should be matched.
• replacement represents the new value that should replace the matched pattern.
• --dir path/to/directory specifies the directory where the search and replace operation should be performed.
• --iglob '**/*.{js,json' is the case-insensitive glob pattern for filtering files. It matches any file with a .js or .json extension.

Example Output:

Initializing fastmod with 4 workers
Searching for regex in files within the specified directory...
Found 2 matches in 2 files.
Replacing regex with replacement...
Replaced 2 occurrences in 2 files.

Use Case 4: Replace for an exact string in .js or .json files

Code:

fastmod --fixed-strings exact_string replacement --extensions json,js

Motivation: This command is useful when you want to replace an exact string in .js or .json files.

Explanation:

• --fixed-strings flag is used to treat the provided pattern as an exact string instead of a regular expression.
• exact_string represents the exact string that should be matched.
• replacement represents the new value that should replace the matched exact string.
• --extensions json,js specifies the file extensions to consider (.js and .json in this case).

Example Output:

Initializing fastmod with 4 workers
Searching for exact_string in .js and .json files...
Found 3 matches in 2 files.
Replacing exact_string with replacement...
Replaced 3 occurrences in 2 files.

Use Case 5: Replace for an exact string without prompt for a confirmation (disables regular expressions)

Code:

fastmod --accept-all --fixed-strings exact_string replacement

Motivation: This command is useful when you want to replace an exact string without being prompted for a confirmation, and without using regular expressions.

Explanation:

• --accept-all flag is used to skip the confirmation prompts and automatically replace the exact string.
• --fixed-strings flag is used to treat the provided pattern as an exact string instead of a regular expression.
• exact_string represents the exact string that should be matched.
• replacement represents the new value that should replace the matched exact string.

Example Output:

Initializing fastmod with 4 workers
Searching for exact_string in all files...
Found 5 matches in 3 files.
Replacing exact_string with replacement without confirmation...
Replaced 5 occurrences in 3 files.

Use Case 6: Replace for an exact string without prompt for a confirmation, printing changed files

Code:

fastmod --accept-all --print-changed-files --fixed-strings exact_string replacement

Motivation: This command allows you to replace an exact string without a confirmation prompt, while also printing the names of the changed files.

Explanation:

• --accept-all flag is used to skip the confirmation prompts and automatically replace the exact string.
• --print-changed-files flag is used to display the names of the changed files in the output.
• --fixed-strings flag is used to treat the provided pattern as an exact string instead of a regular expression.
• exact_string represents the exact string that should be matched.
• replacement represents the new value that should replace the matched exact string.

Example Output:

Initializing fastmod with 4 workers
Searching for exact_string in all files...
Found 7 matches in 4 files.
Replacing exact_string with replacement without confirmation...
Replaced 7 occurrences in 4 files.
Changed files:
- file1.js
- file2.js
- file3.json

In conclusion, Fastmod is a versatile and powerful code replacement tool that provides a simple and efficient way to replace code in a codebase. By exploring these different use cases, you can make the most out of Fastmod to save time and effort in modifying your code.