Modernized version of https://github.com/gagan-bansal/geojson2poly that can be used in a typescript file. It returns the poly fine as a string based on a input GeoJson.

geojsonToPoly.test.ts

import { describe, expect, it } from 'bun:test'
import type { Feature, FeatureCollection, Polygon } from 'geojson'
import { geojsonToPoly } from './geojsonToPoly'

describe('geojsonToPoly', () => {
  it('should convert a simple Polygon GeoJSON to a polyline string', () => {
    const geojson: Feature<Polygon, {}> = {
      type: 'Feature',
      properties: {},
      geometry: {
        type: 'Polygon',
        coordinates: [
          [
            [0, 0],
            [1, 0],
            [1, 1],
            [0, 1],
            [0, 0],
          ],
        ],
      },
    }

    const result = geojsonToPoly(geojson, 'test')
    const expected = 'test-0\n1\n\t0\t0\n\t1\t0\n\t1\t1\n\t0\t1\n\t0\t0\nEND\nEND\n'

    expect(result).toBe(expected)
  })

  it('should handle a FeatureCollection with multiple Polygons', () => {
    const geojson: FeatureCollection<Polygon, {}> = {
      type: 'FeatureCollection',
      features: [
        {
          type: 'Feature',
          properties: {},
          geometry: {
            type: 'Polygon',
            coordinates: [
              [
                [0, 0],
                [1, 0],
                [1, 1],
                [0, 1],
                [0, 0],
              ],
            ],
          },
        },
        {
          type: 'Feature',
          properties: {},
          geometry: {
            type: 'Polygon',
            coordinates: [
              [
                [2, 2],
                [3, 2],
                [3, 3],
                [2, 3],
                [2, 2],
              ],
            ],
          },
        },
      ],
    }

    const result = geojsonToPoly(geojson, 'test')
    const expected =
      'test-0\n1\n\t0\t0\n\t1\t0\n\t1\t1\n\t0\t1\n\t0\t0\nEND\nEND\n' +
      'test-1\n1\n\t2\t2\n\t3\t2\n\t3\t3\n\t2\t3\n\t2\t2\nEND\nEND\n'

    expect(result).toBe(expected)
  })

  it('should throw an error for invalid GeoJSON', () => {
    const invalidGeojson = null

    // @ts-expect-error we are testing a non TS case here where the input is invalid and the types where ignored
    expect(() => geojsonToPoly(invalidGeojson, 'test')).toThrow('Invalid GeoJSON object')
  })
})

geojsonToPoly.ts

// This file is based on https://github.com/gagan-bansal/geojson2poly/blob/master/geojson2poly.js
// It is modernized and external packages are inlined.
import type { Feature, FeatureCollection, Geometry, Polygon } from 'geojson'

/**
 * Extracts geometries of a specific type from a GeoJSON object.
 *
 * @param {FeatureCollection | Feature | Geometry} geojson - The GeoJSON object.
 * @param {string} type - The type of geometry to extract (e.g., 'Polygon').
 * @returns {Array<Geometry>} - An array of extracted geometries.
 */
function extract(geojson: FeatureCollection | Feature | Geometry, type: string): Geometry[] {
  const geometries: Geometry[] = []

  function extractFromFeature(feature: Feature): void {
    if (feature.geometry && feature.geometry.type === type) {
      geometries.push(feature.geometry)
    } else if (feature.geometry && feature.geometry.type === 'GeometryCollection') {
      feature.geometry.geometries.forEach((geometry: Geometry) => {
        if (geometry.type === type) {
          geometries.push(geometry)
        }
      })
    }
  }

  if (geojson.type === 'FeatureCollection') {
    geojson.features.forEach(extractFromFeature)
  } else if (geojson.type === 'Feature') {
    extractFromFeature(geojson)
  } else if (geojson.type === type) {
    geometries.push(geojson)
  }

  return geometries
}

/**
 * Converts a GeoJSON object to a polyline string.
 *
 * @param {FeatureCollection | Feature | Geometry} geojson - The GeoJSON object.
 * @param {string} name - The base name for the polyline entries.
 * @returns {string} - The polyline string.
 */
export function geojsonToPoly(
  geojson: FeatureCollection | Feature | Geometry,
  name: string,
): string {
  if (!geojson || typeof geojson !== 'object') {
    throw new Error('Invalid GeoJSON object')
  }

  const polies = extract(geojson, 'Polygon') as Polygon[]
  let result = ''

  polies.forEach((poly, ind) => {
    result += `${name}-${ind}\n1\n`
    poly.coordinates[0].forEach((p: number[]) => {
      result += `\t${p.join('\t')}\n`
    })
    result += 'END\nEND\n'
  })

  return result
}