How to mock 'readline.createInterface' in jest tests - node.js

Question

I need to test 'readline.createInterface'.
Below is the code that I need to test:
private createReadStreamSafe(filePath: string): Promise<fs.ReadStream> {
return new Promise((resolve, reject) => {
const fileStream = fs.createReadStream(filePath)
console.log('file Stream')
fileStream
.on('error', () => {
reject('create read stream error')
})
.on('open', () => {
resolve(fileStream)
})
})
}
async start() {
const fileStream = await this.createReadStreamSafe(this.filePath)
const rl = readline.createInterface({
input: fileStream,
output: process.stdout,
terminal: false
})
for await (const line of rl) {
...
}
}
I tried following code:
it('should work', async () => {
const mockedReadStream = new Readable()
jest.spyOn(fs, 'createReadStream').mockReturnValue(mockedReadStream as any)
jest.spyOn(readline, 'createInterface').mockImplementation(() => {
const lines = ['text', 'text2', 'text3']
return {
[Symbol.asyncIterator]() {
return {
i: 0,
next: () => {
if (this.i < 3) {
return Promise.resolve({ value: lines[this.i++], done: false })
}
return Promise.resolve({ done: true })
}
}
}
} as any
})
const app = new App('myFile.txt')
let promise = app.start()
mockedReadStream.emit('open')
await expect(promise).resolves.toBe(undefined)
})
But following code is never reached
for await (const line of rl) {
...
}
Is there a way to mock readline.createInterface and then it works with the for await (const line of rl)?

Answer 1

The issue is: the async iterable object is not triggered during the test.
Solution, we can just use an array in the mock, like this:
jest.spyOn(readline, 'createInterface').mockImplementationOnce(() => {
return ['text1', 'text2'] as any
})
Since for await (const item of iterable) works for async iterable objects as well sync iterables. With sync iterables, they will be executed automatically.