Customizing Rolldown Options

tsdown uses Rolldown as its core bundling engine. This allows you to easily pass or override options directly to Rolldown, giving you fine-grained control over the bundling process.

For a full list of available Rolldown options, refer to the Rolldown Config Options documentation.

Overriding inputOptions

You can override the inputOptions generated by tsdown to customize how Rolldown processes your input files. There are two ways to do this:

Using an Object

You can directly pass an object to override specific inputOptions:

tsdown.config.ts

import { defineConfig } from 'tsdown'

export default defineConfig({
  inputOptions: {
    cwd: './custom-directory',
  },
})

In this example, the cwd (current working directory) option is set to ./custom-directory.

Using a Function

Alternatively, you can use a function to dynamically modify the inputOptions. The function receives the generated inputOptions and the current format as arguments:

tsdown.config.ts

import { defineConfig } from 'tsdown'

export default defineConfig({
  inputOptions(inputOptions, format) {
    inputOptions.cwd = './custom-directory'
    return inputOptions
  },
})

This approach is useful when you need to customize options based on the output format or other dynamic conditions.

Overriding outputOptions

The outputOptions can be customized in the same way as inputOptions. For example:

Using an Object

You can directly pass an object to override specific outputOptions:

tsdown.config.ts

import { defineConfig } from 'tsdown'

export default defineConfig({
  outputOptions: {
    comments: 'preserve-legal',
  },
})

In this example, the comments: 'preserve-legal' option ensures that legal comments (e.g., license headers) are preserved in the output files.

Using a Function

You can also use a function to dynamically modify the outputOptions. The function receives the generated outputOptions and the current format as arguments:

tsdown.config.ts

import { defineConfig } from 'tsdown'

export default defineConfig({
  outputOptions(outputOptions, format) {
    if (format === 'esm') {
      outputOptions.comments = 'preserve-legal'
    }
    return outputOptions
  },
})

This ensures that legal comments are preserved only for the esm format.

When to Use Custom Options

While tsdown exposes many common options directly, there may be cases where certain Rolldown options are not exposed. In such cases, you can use the inputOptions and outputOptions overrides to directly set these options in Rolldown.

TIP

Using inputOptions and outputOptions gives you full access to Rolldown's powerful configuration system, allowing you to customize your build process beyond what tsdown exposes directly.