npm install @eggjs/view-nunjucks

import nunjucksPlugin from '@eggjs/view-nunjucks';

export default {
  ...nunjucksPlugin(),
};

import { defineConfig } from 'egg';

export default defineConfig({
  view: {
    defaultViewEngine: 'nunjucks',
    mapping: {
      '.nj': 'nunjucks',
    },
  },
});

// {app_root}/app/controller/test.ts
import { Controller } from 'egg';

class TestController extends Controller {
  async list() {
    const ctx = this.ctx;
    // ctx.body = await ctx.renderString('{{ name }}', { name: 'local' });
    // not need to assign `ctx.render` to `ctx.body`
    // https://github.com/mozilla/nunjucks/blob/6f3e4a36a71cfd59ddc8c1fc5dcd77b8c24d83f3/nunjucks/src/environment.js#L318
    await ctx.render(
      'test.nj',
      { name: 'view test' },
      {
        path: '***',
      }
    );
  }
}

// {app_root}/app/extend/filter.ts
export const hello = (name: string) => `hi, ${name}`;

// so you could use it at template
// {app_root}/app/controller/test.ts
import { Controller } from 'egg';

class TestController extends Controller {
  async list() {
    const ctx = this.ctx;
    ctx.body = await ctx.renderString(
      '{{ name | hello }}',
      { name: 'egg' },
      {
        path: '***',
      }
    );
  }
}

// {app_root}/app/extend/application.ts
import type { Application, ILifecycleBoot } from 'egg';
import markdown from 'nunjucks-markdown';
import { marked } from 'marked';

export default class AppBoot implements ILifecycleBoot {
  app: Application;
  constructor(app: Application) {
    this.app = app;
  }

  async didLoad(): Promise<void> {
    markdown.register(app.nunjucks, marked);
  }
}