node - using jest with esm package

Question

I was wondering how I would incorporate the esm package https://www.npmjs.com/package/esm with jest on a node backend.

I tried setting up a setup file with require("esm") and require("esm")(module) at the very top of the file, but it's still giving me the SyntaxError: Unexpected token error.

I would have previously used node -r esm but jest doesn't support this.

Answer 1

When you perform require("esm")(module), think of it as you are creating an esm-transformer function that is pending a file to be transformed into an ES module.

Here's my attempt with node v8+ with:

• default jest configuration

• default esm configuration

utils-1.js:

export const add = (a, b) => a + b;

utils-2.js:

export const multiAdd = array => array.reduce((sum, next) => sum + next, 0)

_test_/utils-1.assert.js

import { add } from '../utils-1';

describe('add(a,b)', () => {
  it('should return the addtion of its two inputs', () => {
    expect(add(1,2)).toBe(3);
  });
});

_test_/utils-2.assert.js

import { multiAdd } from '../utils-2';

describe('multiAdd(<Number[]>)', () => {
  it('should return a summation of all array elements', () => {
    expect(multiAdd([1,2,3,4])).toBe(10);
  })
});

_test_/utils.test.js

const esmImport = require('esm')(module);
const utils_1 = esmImport('./utils-1.assert')
const utils_2 = esmImport('./utils-2.assert')

Hope this helps!