[javascript] When should I use curly braces for ES6 import?

It seems to be obvious, but I found myself a bit confused about when to use curly braces for importing a single module in ES6. For example, in the React-Native project I am working on, I have the following file and its content:

File initialState.js

var initialState = {
    todo: {
        todos: [
            {id: 1, task: 'Finish Coding', completed: false},
            {id: 2, task: 'Do Laundry', completed: false},
            {id: 2, task: 'Shopping Groceries', completed: false},
        ]
    }
};

export default initialState;

In the TodoReducer.js, I have to import it without curly braces:

import initialState from './todoInitialState';

If I enclose the initialState in curly braces, I get the following error for the following line of code:

Cannot read property todo of undefined

File TodoReducer.js:

export default function todos(state = initialState.todo, action) {
    // ...
}

Similar errors also happen to my components with the curly braces. I was wondering when I should use curly braces for a single import, because obviously, when importing multiple component/modules, you have to enclose them in curly braces, which I know.

The Stack Overflow post at here does not answer my question, instead I am asking when I should or should not use curly braces for importing a single module, or I should never use curly braces for importing a single module in ES6 (this is apparently not the case, as I have seen single import with curly braces required).

I would say there is also a starred notation for the import ES6 keyword worth to mention.

If you try to console log Mix:

import * as Mix from "./A";
console.log(Mix);

You will get:

When should I use curly braces for ES6 import?

The brackets are golden when you need only specific components from the module, which makes smaller footprints for bundlers like webpack.

If you think of import as just syntax sugar for Node.js modules, objects, and destructuring, I find it's pretty intuitive.

// bar.js
module = {};

module.exports = {
  functionA: () => {},
  functionB: ()=> {}
};

 // Really all that is is this:
 var module = {
   exports: {
      functionA, functionB
   }
  };

// Then, over in foo.js

// The whole exported object:
var fump = require('./bar.js'); //= { functionA, functionB }
// Or
import fump from './bar' // The same thing - object functionA and functionB properties


// Just one property of the object
var fump = require('./bar.js').functionA;

// Same as this, right?
var fump = { functionA, functionB }.functionA;

// And if we use ES6 destructuring:
var { functionA } =  { functionA, functionB };
// We get same result

// So, in import syntax:
import { functionA } from './bar';

The curly braces ({}) are used to import named bindings and the concept behind it is destructuring assignment

A simple demonstration of how import statement works with an example can be found in my own answer to a similar question at When do we use '{ }' in javascript imports?.

Dan Abramov's answer explains about the default exports and named exports.

Which to use?

Quoting David Herman: ECMAScript 6 favors the single/default export style, and gives the sweetest syntax to importing the default. Importing named exports can and even should be slightly less concise.

However, in TypeScript named export is favored because of refactoring. Example, if you default export a class and rename it, the class name will change only in that file and not in the other references, with named exports class name will be renamed in all the references. Named exports is also preferred for utilities.

Overall use whatever you prefer.

Additional

Default export is actually a named export with name default, so default export can be imported as:

import {default as Sample} from '../Sample.js';

The curly braces are used only for import when export is named. If the export is default then curly braces are not used for import.

In order to understand the use of curly braces in import statements, first, you have to understand the concept of destructuring introduced in ES6

1. Object destructuring

   var bodyBuilder = {
     firstname: 'Kai',
     lastname: 'Greene',
     nickname: 'The Predator'
   };
   
   var {firstname, lastname} = bodyBuilder;
   console.log(firstname, lastname); // Kai Greene
   
   firstname = 'Morgan';
   lastname = 'Aste';
   
   console.log(firstname, lastname); // Morgan Aste
   

2. Array destructuring

   var [firstGame] = ['Gran Turismo', 'Burnout', 'GTA'];
   
   console.log(firstGame); // Gran Turismo
   

   Using list matching

     var [,secondGame] = ['Gran Turismo', 'Burnout', 'GTA'];
     console.log(secondGame); // Burnout
   

   Using the spread operator

   var [firstGame, ...rest] = ['Gran Turismo', 'Burnout', 'GTA'];
   console.log(firstGame);// Gran Turismo
   console.log(rest);// ['Burnout', 'GTA'];
   

Now that we've got that out of our way, in ES6 you can export multiple modules. You can then make use of object destructuring like below.

Let's assume you have a module called module.js

    export const printFirstname(firstname) => console.log(firstname);
    export const printLastname(lastname) => console.log(lastname);

You would like to import the exported functions into index.js;

    import {printFirstname, printLastname} from './module.js'

    printFirstname('Taylor');
    printLastname('Swift');

You can also use different variable names like so

    import {printFirstname as pFname, printLastname as pLname} from './module.js'

    pFname('Taylor');
    pLanme('Swift');

Usually when you export a function you need to use the {}.

If you have

export const x

you use

import {x} from ''

If you use

export default const x

you need to use

import x from ''

Here you can change X to whatever variable you want.

Summary ES6 modules:

Exports:

You have two types of exports:

1. Named exports
2. Default exports, a maximum one per module

Syntax:

// Module A
export const importantData_1 = 1;
export const importantData_2 = 2;
export default function foo () {}

Imports:

The type of export (i.e., named or default exports) affects how to import something:

1. For a named export we have to use curly braces and the exact name as the declaration (i.e. variable, function, or class) which was exported.
2. For a default export we can choose the name.

Syntax:

// Module B, imports from module A which is located in the same directory

import { importantData_1 , importantData_2  } from './A';  // For our named imports

// Syntax single named import:
// import { importantData_1 }

// For our default export (foo), the name choice is arbitrary
import ourFunction from './A';

Things of interest:

1. Use a comma-separated list within curly braces with the matching name of the export for named export.
2. Use a name of your choosing without curly braces for a default export.

Aliases:

Whenever you want to rename a named import this is possible via aliases. The syntax for this is the following:

import { importantData_1 as myData } from './A';

Now we have imported importantData_1, but the identifier is myData instead of importantData_1.

For a default export we do not use { } when we import.

For example,

File player.js

export default vx;

File index.js

import vx from './player';

File index.js

File player.js

If we want to import everything that we export then we use *:

• If there is any default export in the file, there isn't any need to use the curly braces in the import statement.

• if there are more than one export in the file then we need to use curly braces in the import file so that which are necessary we can import.

• You can find the complete difference when to use curly braces and default statement in the below YouTube video (very heavy Indian accent, including rolling on the r's...).

  21. ES6 Modules. Different ways of using import/export, Default syntax in the code. ES6 | ES2015