Code-Modifier for Typescript based projects.

• Installation
• Command Line Usage
• Example
• Custom Transformation
• Post Transformation
• Builtin Transformations

npm i -g ts-codemod

A typical command looks like -

ts-codemod --transformation [transformation name] --params [transformation params] --write [glob pattern]

So lets say I want to update the import statements throughout the application from something like —

import * as components from '../../../component'

to something like —

import * as components from 'component'

Here I have removed the unnecessary ../../../ from the import statement. To achieve this goal I can use the [normalize-import-path] transformation.

1. Create a .tscodemodrc file

{
  // name of the transformation
  transformation: 'normalize-import-path',

  // transformation params
  params: {
    module: 'component'
  }
}

2. Run the code mod.

ts-codemod --write src/**/*.ts

Alternatively you can also pass all the arguments without creating a .tscodemodrc file —

ts-codemod --transformation normalize-import-path --params.module=component --write src/**/*.ts

Writing a custom transformation isn't very easy and one needs to understand how typescript internally converts plain string to an AST.

A good starter could be to checkout the [transformations] directory. Those transformations are written for a varied level of complexity. Also checkout the AST Explorer website to get an understanding of ASTs in general.

A custom transformation (my-custom-transformation.ts) can be implemented via extending the Transformation class.

import * as ts from 'typescript'
import {Transformation} from 'ts-codemod'

// my-custom-transformation.ts
export default class MyCustomTransformation extends Transformation {
  visit(node: ts.Node): ts.VisitResult<ts.Node> {
    // write your implementation here
    return node // will apply no-change
  }
}

It can then be executed as —

ts-codemod -t ./my-custom-transformation.ts src/**.ts

Passing Custom Params: To pass custom params to your transformation can be done as follows —

export type MyParams = {
  moduleName: string
}

// my-custom-transformation.ts
export default class MyCustomTransformation extends Transformation<MyParams> {

  // Called before the transformation is applied on the file
  before () {

  }

  visit(node: ts.Node): ts.VisitResult<ts.Node> {


    // access the params
    console.log(this.params.moduleName)

    ...
  }

  // Called after the transformation is applied on the file
  after () {

  }
}

The additional params are passed via the --params.moduleName cli argument or if you are using a .tscodemodrc file —

{
  params: {
    moduleName: 'abc'
  }
}

1. Life can't be that simple right? Running transformations will generally ruin the formatting of your files. A recommended way to solve that problem is by using Prettier.
2. Even after running prettier its possible to have unnecessary new lines added/removed. This can be solved by ignoring white spaces while staging the changes in git.

git diff --ignore-blank-lines | git apply --cached