esbuild-plugin-eval

esbuild-plugin-eval (for Node.JS)

This is an esbuild plugin that evaluates a module before importing it. It's useful in cases where you want to render static parts of your application at build time to prune runtime dependencies, such as pre-rendering html from JSX, or pre-calculating CSP header hashes.

This plugin will evaluate any imported module with .eval before the extension (example.eval.ts or example.eval.jsx and so on). It does this by bundling the module into a data url, dynamically importing it, and then re-exporting the results.

Usage

Install the dependency:

npm install esbuild-plugin-eval --save-dev

yarn add esbuild-plugin-eval -D

Add it to your esbuild plugins:

import { build } from 'esbuild'
import evalPlugin from 'esbuild-plugin-eval'

await build({
  ...yourConfig
  plugins: [evalPlugin],
})

Example input:

// index.js (entry point)
export * from './schema.eval.js'

// schema.eval.js
import { z } from 'zod'
import { zodToJsonSchema } from 'zod-to-json-schema'

const mySchema = z
  .object({
    myString: z.string().min(5),
    myUnion: z.union([z.number(), z.boolean()]),
  })
  .describe('My neat object schema')

export const jsonSchema = zodToJsonSchema(mySchema, 'mySchema')

Example after building:

// build/index.js

var jsonSchema = { "$ref": "#/definitions/mySchema", "definitions": { "mySchema": { "type": "object", "properties": { "myString": { "type": "string", "minLength": 5 }, "myUnion": { "type": ["number", "boolean"] } }, "required": ["myString", "myUnion"], "additionalProperties": false, "description": "My neat object schema" } }, "$schema": "http://json-schema.org/draft-07/schema#" };

In this case, we generate JSON schema at build time, and then serve it as a static file at runtime. The two dependecies used to create the schema, namely zod and zod-to-json-schema, are not included in the final bundle, thus reducing its size from 299KB to just 712 bytes.

Caveats

A best effort is made to properly handle function exports, but keep in mind that all variables accessed from exported functions will need to be exported as well.

So this won't work:

let message = 'Hello, world!'

export default () => console.log(message)

But this will:

export let message = 'Hello, world!'
//^^^^
export default () => console.log(message)

Thanks to @jed for the original Deno implementation.