JavaScript export default in javascript export default javascript object,export default javascript es6,export default

I'm working through a tutorial on how to create a restful api using express and mongoose.

I'm understanding everything except for this bit right here

import mongoose from 'mongoose';
import config from './config';

export default callback => {
    // connect to the database
    let db = mongoose.connect(config.mongoUrl);

    // callback db
    callback(db);
}
Answer:1

callback isn't the function's name in this case. export default is exporting an anonymous arrow function which takes callback as an argument. Since it only has one argument, they don't need to be wrapped in parens for it to be valid JavaScript. When a function is passed in, it will call it asynchronously, returning the data passed to the callback function. To illustrate, consider the following example.

import mongoose from 'mongoose';
import config from './config';

export default callback => {
    // connect to the database
    setTimeout( function () {
        let db = mongoose.connect(config.mongoUrl);

        callback(db);
    }, 1000); // simulate latency

}

And then, when you import the file elsewhere:

import db from './db';

db( database => {
    // One second later, this function would be called. 
    // Do something with the database here.
});

Hopefully this example illustrates this concept clearly.

Answer:2

This is a single param arrow function, check out its description on mdn

// Parentheses are optional when there's only one parameter:
(singleParam) => { statements }
singleParam => { statements }

MDN source

Answer:3

That code uses arrow function. So it's the same as this:

import mongoose from 'mongoose';
import config from './config';

export default function(callback) {
    // connect to the database
    let db = mongoose.connect(config.mongoUrl);

    // callback db
    callback(db);
}

so this function is called back by some other module using a callback function as a parameter which is the runned with callback(db)

Hope it's clearer now.

Answer:4

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