为什么等待代码在同步代码之前执行？

Question

I have a signup user function that I dont really understand how it's working...

module.exports.signupUser = async (req, res) => {
let email = req.body.email;
let password = req.body.password;
if(!validator.isEmail(email))
{
    res.status(400).json({
        "message": "Please enter a valid email!"
    }).end();
}
else if(!validator.isLength(password, {min:6})){
    res.status(400).json({
        "message": "Password must have at least 6 characters!"
    }).end();
}
else {
    const salt = bcrypt.genSaltSync(10);
    const hashedPassword = await bcrypt.hash(password, salt);
    let params = [email, hashedPassword];
    console.log(hashedPassword);

    let query = 'insert into users (email, password) values (?,?)';
    connection.query(query, params, (err, result, fields) => {
        if(err && err.code === 'ER_DUP_ENTRY') {
            res.status(400).json({
                "message": "There is an account already associated with this email adress!"
            }).end();
        }
        else {
            res.status(200).json({
                "message": "User created!"
            }).end();
        }
    });
}   

}

So in the last else, I use await bcrypt.hash(password, salt) to encrypt my password. I'm new to JS but still I understand that await executes code asyncronously, so console.log(hashedPassword) should not return me the hashed password because this log will execute before the actual hashing. Anyone can explain me what is really happening?

Answer 1

const hashedPassword = await bcrypt.hash(password, salt);
console.log(hashedPassword);

If you use the await keyword, nodejs will wait for promise to resolve before moving to the next line in the async function. Your console.log's result will be the hashed password.

If you don't use the await keyword, node will start resolving the promise but will not wait for it to finish resolving before moving to next line. Your console.log's result will be Promise { pending }

Answer 2

await executes the code synchronously. So in your case await will wait for the execution of bcrypt.hash .

For more clarity bcrypt.hash function returns a promise and now we have two choices here.

1. Await for the promise to resolve

const hashedPassword = await bcrypt.hash(password, salt);
console.log(hashedPassword);

the hashedPassword will contain the actual output we need.

2. Handle promise resolution using then

bcrypt.hash(password, salt).then(hashedPassword => {
    console.log(hashedPassword);
})

As the bcrypt.hash function returns a promise, we need to handle the rest of the code inside a then block as it'll only return the value once the function resolves the promise.

In case you want to execute your code asynchronously , you need to remove the await keyword which waits for the promise to resolve before moving to the next line.