Extract part of a template

Use eject to take one marked region out of a larger template file.

Rendered from docs/how-to/extract-part-of-a-template.md in the generator repository — where a correction belongs, and where the test suite runs every example on this page or states why it does not.

Sometimes the file you want to borrow from is a working file—one that compiles, or runs, or that somebody maintains for its own sake—and you want one region of it. eject takes the region between two markers and discards everything else, markers included.

The template is tpl/example.js, a file that works on its own:

import { setup } from './harness.js'

// EXTRACT-START
export function handler(req) {
  return { ok: true }
}
// EXTRACT-END

setup(handler)

Take just the middle:

import { Jostraca, Project, File, Fragment } from 'jostraca'

await Jostraca().generate({ folder: './out' }, () => {
  Project({}, () => {
    File({ name: 'handler.js' }, () => {
      Fragment({
        from: '../tpl/example.js',
        eject: ['// EXTRACT-START\n', '// EXTRACT-END\n'],
      })
    })
  })
})

The generated handler.js:

export function handler(req) {
  return { ok: true }
}

The markers themselves are removed, so the extracted region needs no cleaning up afterwards.

eject is forgiving in one direction only, and silently: if either marker is missing from the file, or the end marker comes before the start, or you pass a one-element array, the whole source is used instead. There is no error. Check the output the first time, and pin it with a test if the template is maintained by somebody else.

Both markers may be regular expressions instead of strings, which is what you want when the region boundary varies.

Extract part of a template

See also#