javascript保存功能用法

Question

I am learning about building a new user login app and there is a line of code the makes me quite confused and Google did not show me any enlightening result.

 module.exports.createUser = function(newUser, callback){ bcrypt.genSalt(10, function(err, salt) { bcrypt.hash(newUser.password, salt, function(err, hash) { newUser.password = hash; newUser.save(callback); }); }); } 

It is quite straightforward: The new user module is trying to crypt the password and runs the callback(It is my understanding). But this line here:

 newUser.save(callback); 

It is very confusing to me and would anyone explain what it does here? Thank you

Answer 1

You pass a callback function as a parameter to the outer most function. And when the genSalt- and hash-functions are done, you pass the same callback function as a parameter to newUser.save() . It is a function that (probably) will be executed inside newUser.save() .

Sometimes using callbacks are a way to say that some code has completed, and then run a new function. Useful for async funcions.

A simpler example:

serveCake = function(){ // this is my callback function
 // serve the cake
}

createCake = function(ingredients, callback){ // pass serveCake as a callback here.
 // Mix the ingredients and put the cake in the oven
 // When the cake is finished baking, serve it
 callback(); // call the callback function.
}
const ingredients = ['suger', 'butter'];

createCake(ingredients,serveCake);

The same happens with your code, but the callback function will (probably) be executed inside the newUser.save() -method.

Answer 2

Mongoose save method expects an optional callback function as an argument ( Reference Doc ). So when you are calling newUser.save(callback); you are passing the same callback function that you got in the createUser(newUser, callback); so whichever method will call this method ie suppose in following code callIt() function is calling your code.

module.exports.createUser = (newUser, callback) => {
  bcrypt.genSalt(10, (err, salt) => {
    bcrypt.hash(newUser.password, salt, (err, hash) => {
      newUser.password = hash;
      newUser.save(callback);
    });
  });
};

function callIt() {
    module.exports.createUser({ name: 'ridham' }, function (err, user) {
        if (err) {
            console.log(error);
        } else {
            console.log(user);
        }
    });
}    

callIt();

so here you are giving function (err, user) {...} function as an argument which can be called by invoked function to pass back the access to invoking the function(handling async nature of JS). and your invoke function again invoking another function passing the same callback reference so basically, you are chaining the same callback.

Callback Explanations can be found here and here .