---
title: "Replace markers in a template"
description: "Swap named placeholders in a template for strings, computed values or generated components."
source: "https://jostraca.org/how-to/replace-markers-in-a-template/"
---

# Replace markers in a template

Swap named placeholders in a template for strings, computed values or generated components.

Rendered from [`docs/how-to/replace-markers-in-a-template.md`](https://github.com/jostraca/jostraca/blob/master/docs/how-to/replace-markers-in-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.

`$$path$$` covers values that live in the model. For everything else—a name computed at generate time, a whole block of generated code—pass a `replace` map. It works on `Fragment`, `Content` and `Copy`.

The template is `tpl/class.js`:

```js
export class CLASS_NAME {
  // #Body
}
```

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

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

      Fragment({
        from: '../tpl/class.js',
        replace: {
          CLASS_NAME: 'Widget',
          '#Body': () => Content('    return 1\n'),
        },
      })
    })
  })
})
```

The generated `widget.js`:

```js
export class Widget {
    return 1
}
```

Two different key forms did the work there, and the difference matters:

-   **A plain key is matched literally.** `CLASS_NAME` replaced the text wherever it appeared.
-   **A key starting `#` is a comment tag.** `'#Body'` matched the whole line `// #Body`, indentation included, and replaced it. Writing `#Body` bare in the template would not have matched.

A key wrapped in slashes is a regular expression: `'/name_\\w+/'`. A function value receives the named capture groups, the whole match under `$&`, and the current `indent`.

One asymmetry to know before you rely on it. A replacement that **emits a component** lands in the right place inside a `Fragment`, which streams its output, but is appended after the whole string inside a `Content`, which joins first. Where position matters, use `Fragment`—or return a string.

## See also

-   [Utilities reference](https://jostraca.org/docs/reference-utilities#template) for the full `replace` grammar.
-   [Fill a template file’s slots](https://jostraca.org/how-to/fill-a-template-slot) for regions big enough to deserve a name.