await expect(xxx) vs expect(await yyy)

[SiegfriedBz]

Can someone please explain the differences between
await expect(xxx)
e.g. : expect(await ethers.provider.getBalance(contract.address)).to...

expect(await yyy)
e.g. : await expect(contract.contractMethod()).to...
?
Thanks in advance!

[pacelliv]

Hello @SiegfriedBz

The await keyword is used to wrap an expression that returns a Promise to wait for its fulfillment. Not everything returns a Promise.

A. In this example:

 expect(await ethers.provider.getBalance(contract.address)).to...

The method provider.getBalance() from the ethers.js library returns a Promise. Thus requiring an await keyword to wait for this Promise to be fulfilled. But the expect from the assertion library of Chai library doesn't return a Promise. That's why the await is inside the parenthesis.

B. In this example:

await expect(contract.contractMethod()).to...

I'm guessing a Chai Matcher with an Emitting event was used. That's why the entire expression is wrapped with the await keyword for a correct execution.

I recommend you to read about Promises, await/Async functions and APIs or find a FCC course about these topics.

[krakxn]

@SiegfriedBz

It is really straight-forward actually.

await expect(xxx)

Here, it does not wait for xxx to evaluate, but it waits for the expect to evaluate before proceeding further to the next lines.

expect(await yyy)

Here, it waits for yyy to evaluate before proceeding, but does not wait for the expect to evaluate before proceeding.

---

Now, you will be in a dilemma as to which is better. (Hint: await expect(xxx) is the better choice! await can be used both ways as well, if xxx takes time to evaluate)

This is a good read to further clear your doubts: https://stackoverflow.com/a/66069510