Can you explain the functionality of sinon's stub.yields method?

Question

Can you explain the functionality of sinon's stub.yields method?

The explanation given in the documentation for sinon regarding stub.yields is as follows:

By using stub.yields([arg1, arg2, ...]), you are essentially performing a function similar to callsArg.

This will result in the stub executing the first callback it receives with the specified arguments (if any).

In scenarios where a method has multiple callbacks, yieldsRight must be used to call the last callback or callsArg should be utilized to have the stub trigger callbacks other than the first or last one.

After reading this particular section multiple times, it still remains unclear and especially confusing when trying to comprehend the emphasized part.

To aid my understanding, I require a more detailed explanation along with a couple of examples demonstrating the usage of yields (which the official documentation fails to provide).

javascript sinon

Answer 1

Answer №1

When you are stubbing a function that requires a callback, such as an asynchronous database request, it allows the stub to mimic the results that the function would typically provide to your callback.

Let's make this easier to understand with an example:

// simulated db api
let db = {
  get(query, cb) {
    cb(null, "your results from the query")
  }
}

function runQuery(q) {
  db.get(q, (err, val) => {
    if (err) console.log("error!", err)
    else console.log("value:", val)
  })
}
// call it normally
runQuery("some query")

// stub the DB get method
let stub = sinon.stub(db, 'get');

// fake query results
stub.yields(null, "results from Sinon Stub")

// now stubbed
runQuery("some query")
// assert that `runQuery` did what it should
// given a value of `results from Sinon Stub`
// from db.get

// see how it handles an error:
stub.yields("Some error")

runQuery("some query")
// assert that `runQuery` did what it should
// when db errors with "Some error"

<script src="https://cdnjs.cloudflare.com/ajax/libs/sinon.js/7.3.2/sinon.min.js"></script>

This technique is useful for testing an asynchronous function that uses callbacks and verifying different outcomes. For instance, if you were working with a database function that required a callback like this:

db.get("someVal", (err, val) => {/* do something */}

You could simulate various responses from the database by yielding different values and then perform assertions against your code.

Answer 2

When you are stubbing a function that requires a callback, such as an asynchronous database request, it allows the stub to mimic the results that the function would typically provide to your callback.

Let's make this easier to understand with an example:

// simulated db api
let db = {
  get(query, cb) {
    cb(null, "your results from the query")
  }
}

function runQuery(q) {
  db.get(q, (err, val) => {
    if (err) console.log("error!", err)
    else console.log("value:", val)
  })
}
// call it normally
runQuery("some query")

// stub the DB get method
let stub = sinon.stub(db, 'get');

// fake query results
stub.yields(null, "results from Sinon Stub")

// now stubbed
runQuery("some query")
// assert that `runQuery` did what it should
// given a value of `results from Sinon Stub`
// from db.get

// see how it handles an error:
stub.yields("Some error")

runQuery("some query")
// assert that `runQuery` did what it should
// when db errors with "Some error"

<script src="https://cdnjs.cloudflare.com/ajax/libs/sinon.js/7.3.2/sinon.min.js"></script>

This technique is useful for testing an asynchronous function that uses callbacks and verifying different outcomes. For instance, if you were working with a database function that required a callback like this:

db.get("someVal", (err, val) => {/* do something */}

You could simulate various responses from the database by yielding different values and then perform assertions against your code.

Can you explain the functionality of sinon's stub.yields method?

Answer №1

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