---
title: "Branch and loop in a generator"
description: "Use ordinary JavaScript control flow to decide what a generator emits."
source: "https://jostraca.org/how-to/branch-and-loop/"
---

# Branch and loop in a generator

Use ordinary JavaScript control flow to decide what a generator emits.

Rendered from [`docs/how-to/branch-and-loop.md`](https://github.com/jostraca/jostraca/blob/master/docs/how-to/branch-and-loop.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.

There is no conditional syntax to learn, because a component call is a function call. Use `if`, use `for`, use `map`.

```js
import { Jostraca, Project, Folder, File, Content } from 'jostraca'

const model = {
  name: 'acme',
  typescript: true,
  routes: ['health', 'login'],
}

await Jostraca().generate({ folder: './out' }, () => {
  Project({ folder: model.name }, () => {

    if (model.typescript) {
      File({ name: 'tsconfig.json' }, () => Content('{}\n'))
    }

    Folder({ name: 'routes' }, () => {
      for (const route of model.routes) {
        File({ name: route + (model.typescript ? '.ts' : '.js') }, () => {
          Content('export const name = "' + route + '"\n')
        })
      }
    })
  })
})
```

```text
acme/routes/health.ts
acme/routes/login.ts
acme/tsconfig.json
```

To skip a component, do not call it. There is no no-op component to substitute—`None` exists internally and is not exported—so the branch is the mechanism.

`each` is available when you want its extras: it takes an object as readily as an array, it can sort, and it wraps scalars so a uniform callback works either way. A plain loop is otherwise fine and reads better.

```js
import { Jostraca, Project, File, Content, each } from 'jostraca'

const services = { beta: { port: 2 }, alpha: { port: 1 } }

await Jostraca().generate({ folder: './out' }, () => {
  Project({}, () => {
    File({ name: 'ports.txt' }, () => {
      each(services, (svc) => {
        Content(svc.key$ + '=' + svc.port + '\n')
      })
    })
  })
})
```

The generated `ports.txt`:

```text
alpha=1
beta=2
```

Note the order. `each` visits object keys **sorted**, not in insertion order, so that the TypeScript and Go implementations produce the same bytes. `svc.key$` is the key, stamped on by `each`.

## See also

-   [Generate one file per item](https://jostraca.org/docs/reference-components#list) using the `List` component, when the repetition is inside one file.