Im trying to use object destructing imports in my code.
I would like to import only map and extend in my component.
Would I do
import { map, extend } from lodash;
How can I use it in my code?
Javascript also has a map, so if I do
let arr = [1,3,4];
arr.map((item) => console.log(item))
How can I refer to lodash map, not the actual map method?
PS: Im trying to understand object destructing methods. I know I can use ES6 map, this is just an example.
This is a default import.
import _ from 'lodash';
_.map(...);
This is a named import.
import { map } from 'lodash';
map(...);
Using map() as a method of the default import and as a standalone function are essentially the same (with the only meaningful difference being the value of this). Which syntax works is up to the library you are importing. You can only use a default import if the library defines a default export and same for named imports / exports. Both can also be used at the same time, though this is a bit unusual.
The default import can be named whatever your heart desires. If you want to confuse people and pretend that Lodash is jQuery, you can do so.
`import $ from 'lodash';`
On the other hand, named imports have specific names that they are bound to. So unlike the default import, trying to use the named import syntax on $ vs _ refers to an entirely different object and will throw if not defined as an export within the library.
// These are very different.
import { $ } from 'lodash';
import { _ } from 'lodash';
If it becomes necessary, you can "rename" a named import.
import { map as crazyFunc } from 'lodash';
In the above case, crazyFunc can be named whatever your heart desires, but map must remain as-is, otherwise it will refer to an entirely different object and will throw if not defined as an export within the library.
It may help to understand that these are equivalent.
import { default as _ } from 'lodash';
import _ from 'lib';
The latter is simply sugar for the former. And the default export syntax is sugar for creating a named export with the name default.
For more info (including features not covered here), see ES6 modules syntax.
Full example:
import { map } from 'lodash';
const squares = map([4, 8], (n) => {
return n * n;
});
console.log(squares); // => [16, 64]
Related
I have referred all the questions in stackoverflow.
But none of the suggested why and when to use default export.
I just saw that default can be metioned "When there is only one export in a file"
Any other reason for using default export in es6 modules?
Some differences that might make you choose one over the other:
Named Exports
Can export multiple values
MUST use the exported name when importing
Default Exports
Export a single value
Can use any name when importing
This article does a nice job of explaining when it would be a good idea to use one over the other.
It's somewhat a matter of opinion, but there are some objective aspects to it:
You can have only one default export in a module, whereas you can have as many named exports as you like.
If you provide a default export, the programmer using it has to come up with a name for it. This can lead to inconsistency in a codebase, where Mary does
import example from "./example";
...but Joe does
import ex from "./example";
In contrast, with a named export, the programmer doesn't have to think about what to call it unless there's a conflict with another identifier in their module.¹ It's just
import { example } from "./example";
With a named export, the person importing it has to specify the name of what they're importing. They get a nice early error if they try to import something that doesn't exist.
If you consistently only use named exports, programmers importing from modules in the project don't have to think about whether what they want is the default or a named export.
¹ If there is a conflict (for instance, you want example from two different modules), you can use as to rename:
import { example as widgetExample } from "./widget/example";
import { example as gadgetExample } from "./gadget/example";
You should almost always favour named exports, default exports have many downsides
Problems with default exports:
Difficult to refactor or ensure consistency since they can be named anything in the codebase other than what its actually called
Difficult to analyze by automated tools or provide code intellisense and autocompletion
They break tree shaking as instead of importing the single function you want to use you're forcing webpack to import the entire file with whatever other dead code it has leading to bigger bundle sizes
You can't export more than a single export per file
You lose faster/direct access to imports
checkout these articles for a more detailed explanation:
https://blog.neufund.org/why-we-have-banned-default-exports-and-you-should-do-the-same-d51fdc2cf2ad
https://humanwhocodes.com/blog/2019/01/stop-using-default-exports-javascript-module/
https://rajeshnaroth.medium.com/avoid-es6-default-exports-a24142978a7a
With named exports, one can have multiple named exports per file. Then import the specific exports they want surrounded in braces. The name of imported module has to be the same as the name of the exported module.
// imports
// ex. importing a single named export
import { MyComponent } from "./MyComponent";
// ex. importing multiple named exports
import { MyComponent, MyComponent2 } from "./MyComponent";
// ex. giving a named import a different name by using "as":
import { MyComponent2 as MyNewComponent } from "./MyComponent";
// exports from ./MyComponent.js file
export const MyComponent = () => {}
export const MyComponent2 = () => {}
You can also alias named imports, assign a new name to a named export as you import it, allowing you to resolve naming collisions, or give the export a more informative name.
import MyComponent as MainComponent from "./MyComponent";
You can also Import all the named exports onto an object:
import * as MainComponents from "./MyComponent";
// use MainComponents.MyComponent and MainComponents.MyComponent2 here
One can have only one default export per file. When we import we have to specify a name and import like:
// import
import MyDefaultComponent from "./MyDefaultExport";
// export
const MyComponent = () => {}
export default MyComponent;
The naming of import is completely independent in default export and we can use any name we like.
From MDN:
Named exports are useful to export several values. During the import, one will be able to use the same name to refer to the corresponding value.
Concerning the default export, there is only a single default export per module. A default export can be a function, a class, an object or anything else. This value is to be considered as the “main” exported value since it will be the simplest to import.
There aren't any definitive rules, but there are some conventions that people use to make it easier to structure or share code.
When there is only one export in the entire file, there is no reason to make it named.
Also, when your module has one main purpose, it could make sense to make that your default export. In those cases you can extra named exports
In react for example, React is the default export, since that is often the only part that you need. You don't always Component, so that's a named export that you can import when needed.
import React, {Component} from 'react';
In the other cases where one module has multiple equal (or mostly equal) exports, it's better to use named exports
import { blue, red, green } from 'colors';
1st Method:-
export foo; //so that this can be used in other file
import {foo} from 'abc'; //importing data/fun from module
2nd Method:-
export default foo; //used in one file
import foo from 'blah'; //importing data/fun from module
3rd Method:-
export = foo;
import * as foo from 'blah';
The above methods roughly compile to the following syntax below:-
//all export methods
exports.foo = foo; //1st method
exports['default'] = foo; //2nd method
module.exports = foo; //3rd method
//all import methods
var foo = require('abc').foo; //1st method
var foo = require('abc')['default']; //2nd method
var foo = require('abc'); //3rd method
For more information, visit to Default keyword explaination
Note:- There can be only one export default in one file.
So whenever we are exporting only 1 function, then it's better to use default keyword while exporting
EASIEST DEFINITION TO CLEAR CONFUSIONS
Let us understand the export methods, first, so that we can analyze ourselves when to use what, or why do we do what we do.
Named exports: One or more exports per module. When there are more than one exports in a module, each named export must be restructured while importing. Since there could be either export in the same module and the compiler will not know which one is required unless we mention it.
//Named export , exporting:
export const xyz = () =>{
}
// while importing this
import {xyx} from 'path'
or
const {xyz} = require(path)
The braces are just restructuring the export object.
On the other hand , default exports are only one export per module , so they are pretty plain.
//exporting default
const xyz =() >{
};
export default xyz
//Importing
import xyz from 'path'
or
const xyz = require(path)
I hope this was pretty simple to understand, and by now you can understand why you import React modules within braces...
Named Export: (export)
With named exports, one can have multiple named exports per file. Then import the specific exports they want surrounded in braces. The name of imported module has to be the same as the name of the exported module.
// imports
// ex. importing a single named export
import { MyComponent } from "./MyComponent";
// ex. importing multiple named exports
import { MyComponent, MyComponent2 } from "./MyComponent";
// ex. giving a named import a different name by using "as":
import { MyComponent2 as MyNewComponent } from "./MyComponent";
// exports from ./MyComponent.js file
export const MyComponent = () => {}
export const MyComponent2 = () => {}
Import all the named exports onto an object:
// use MainComponents.MyComponent and MainComponents.MyComponent2 here
import * as MainComponents from "./MyComponent";
Default Export: (export default)
One can have only one default export per file. When we import we have to specify a name and import like:
// import
import MyDefaultComponent from "./MyDefaultExport";
// export
const MyComponent = () => {}
export default MyComponent;
Note: The naming of import is completely independent in default export and we can use any name we like.
Here's a great answer that explains default and named imports in ES6
We use lodash in an angular app written with typescript. Currently we import lodash as follows:
import * as _ from 'lodash';
//.. code which uses _.pluck()
However, for the sake of tree shaking, we want to change to the following:
import {pluck, delay} from 'lodash';
//.. code which uses _.pluck() needs changed to pluck()
The problem is we need to do a lot of tedious code changes because using the second import method loses the namespace of _ and there could be name conflicts. Is there a way of specifying the things we want to import but maintain the namespace? I was thinking something as follows, but it doesn't work:
import {pluck, delay} as _ from 'lodash';
//.. code which uses _.pluck() needs changed to pluck()
Not directly in import but you can create the _ object manually.
import { pluck, delay } from 'lodash';
const _ = { pluck, delay };
This question already has answers here:
When should I use curly braces for ES6 import?
(11 answers)
Closed 4 years ago.
I am learning Javascript imports and I am yet to understand when we use curly braces while importing items(functions, objects, variables) from another JS file.
import Search from './models/Search';
import * as searchView from './views/searchView';
import { elements, renderLoader } from './views/base'
//elements is an object, renderLoader is a function
The import statements are used to import the exported bindings from another module
The curly braces ({}) are used to import named bindings and the concept behind it is called destructuring assignment The concept of destructuring assignment is a process that makes it possible to unpack the values from arrays or objects into distinct variables in the imported module
The curly braces ({}) are used to import named bindings
I would like to explain different types of imports in ES6 with the help of an example
Suppose we have a module named Animals(Animals.js) let suppose the module exports a default binding Man and several other named bindings such as Cat, Dog etc
/*
Animals.js
*/
..
export Cat;
export Dog
export default Man
Import a single export from a module
In order to export a single export from another module (let's say Cat) we can write it like this
/*
Anothermodule.js
*/
import {Cat} from "./Animals"
Similarly for Dog
/*
YetAnothermodule.js
*/
import {Dog} from "./Animals"
Import multiple exports from module
You can also import multiple modules as follows
/*
Anothermodule.js
*/
import {Dog, Cat} from "./Animals"
Import an export with a more convenient alias
/*
Anothermodule.js
*/
import {Dog as Puppy} from './Animals.js';
Rename multiple exports during import
/*
Anothermodule.js
*/
import {Dog as Puppy, Cat as Kitty} from './Animals.js';
But in the case to import Man into another module since it is a default export you can write it like this
/*
Anothermodule.js
*/
import Man from './Animals.js';
You can also mix both the above variants for example
/*
Anothermodule.js
*/
import Man, {Dog as Puppy, Cat as Kitty} from '/Animals.js';
Import an entire module's contents
If you want to import everything you can use
/*
Anothermodule.js
*/
import * as Animals from './Animals.js';
Here, accessing the exports means using the module name ("Animals" in this case) as a namespace. For example, if you want to use Cat in this case you can use it like below
Animals.Cat
You can read more information about import here
you can read about destructuring here
import { elements, renderLoader } from './views/base'
is the way you need to import single, named exports from a module, in this case it is importing named exports elements and renderLoader from base.js.
The { elements, renderLoader } syntax is in many cases just syntactic sugar (called destructuring) added in recent versions of the ECMAScript standard.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment#Object_destructuring
In this case, though, it is necessary to get only the named exports you want.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import#Import_a_single_export_from_a_module
Please note that you can also pick new names for your variables like this:
import { elements as newNameForElements, renderLoader as newNameForRenderLoader } from './views/base'
which would then make the elements export available as newNameForElements etc.
import Search from './models/Search';
Imports the default exported element as Search.
import * as searchView from './views/searchView';
Imports everything into searchView that has been exported.
import { elements, renderLoader } from './views/base'
Imports a hand-picked number of named exported elements.
{} is used when you want to import part of an object. The * as searchView one will import all properties and methods in the searchView file.
Suppose './views/base' has 3 properties: elements, renderLoader, additionalParam (Assuming that all three have been exported as named exports in the file)
When doing
import { elements, renderLoader } from './views/base'
you import only those 2 specific properties
But when you do
import * as base from './views/base'
you import all three properties in the object named base
Take the following example:
File to be imported, say importedFile.js:
var defaultExport, otherExport1, otherExport2, otherExport3;
export default defaultExport = () => {
console.log("Default Export")
}
export otherExport1 = "Other non-default Export";
export otherExport2 = function() {
console.log("Some more non-default Export");
};
export otherExport3 = { msg: "again non-default Export" };
Now in your main JS file, if you would do the following:
import something from './importedFile.js;
Here the variable something would get the value of the variable/function that has been exported as default in the importedFile.js file, i.e. the variable defaultExport. Now, if you do something like the following:
import { otherExport1, otherExport2 } from './importedFile.js;
It would import specifically otherExport1 and otherExport2 variable and function and not the defaultExport and otherExport3.
You can also do something like the following to import all the variables by their names from importedFile.js:
import { defaultExport, otherExport1, otherExport2, otherExport3 } from './importedFile.js';
Conclusion:
curly braces are used to choose variables/functions/objects (using a technique called object destructuring in ES6) that need to be imported without importing all the other unnecessary exported variables/functions/objects.
If you don't specify curly braces, it would always import only the variable/function/object that has been exported as default and nothing else. It would import undefined if nothing has been exported as default export.
You can use curly braces to import implicitly and selectively from another module functions or objects and so on.
// import implicitly one function and one constant from example.js
import { a, b } from 'example'
example.js
// export a and b but kept c private to example.js
export const a => { ... }
export const b = "hello"
const c = "private, not visible to the outside"
More infos:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import
https://developer.mozilla.org/en-US/docs/web/javascript/reference/statements/export
If something is exported as default it is imported without curly braces.
If multiple variables are exported it is imported using curly braces.
For example,
in somefunction.js
export default module;
import module from './somefunction.js';
in someOtherFunction.js
export func1;
export func2;
import { func1, func2 } from './someOtherFunction.js'
You can export more than 1 content from a single module.
For example at your code:
import * as searchView from './views/searchView'; //1
import { elements, renderLoader } from './views/base' //2
At //1, you import Everything from './views/searchView';
At //2, there might be more content from './views/base', but you import only elements and renderLoader
For more information: import MDN
I have referred all the questions in stackoverflow.
But none of the suggested why and when to use default export.
I just saw that default can be metioned "When there is only one export in a file"
Any other reason for using default export in es6 modules?
Some differences that might make you choose one over the other:
Named Exports
Can export multiple values
MUST use the exported name when importing
Default Exports
Export a single value
Can use any name when importing
This article does a nice job of explaining when it would be a good idea to use one over the other.
It's somewhat a matter of opinion, but there are some objective aspects to it:
You can have only one default export in a module, whereas you can have as many named exports as you like.
If you provide a default export, the programmer using it has to come up with a name for it. This can lead to inconsistency in a codebase, where Mary does
import example from "./example";
...but Joe does
import ex from "./example";
In contrast, with a named export, the programmer doesn't have to think about what to call it unless there's a conflict with another identifier in their module.¹ It's just
import { example } from "./example";
With a named export, the person importing it has to specify the name of what they're importing. They get a nice early error if they try to import something that doesn't exist.
If you consistently only use named exports, programmers importing from modules in the project don't have to think about whether what they want is the default or a named export.
¹ If there is a conflict (for instance, you want example from two different modules), you can use as to rename:
import { example as widgetExample } from "./widget/example";
import { example as gadgetExample } from "./gadget/example";
You should almost always favour named exports, default exports have many downsides
Problems with default exports:
Difficult to refactor or ensure consistency since they can be named anything in the codebase other than what its actually called
Difficult to analyze by automated tools or provide code intellisense and autocompletion
They break tree shaking as instead of importing the single function you want to use you're forcing webpack to import the entire file with whatever other dead code it has leading to bigger bundle sizes
You can't export more than a single export per file
You lose faster/direct access to imports
checkout these articles for a more detailed explanation:
https://blog.neufund.org/why-we-have-banned-default-exports-and-you-should-do-the-same-d51fdc2cf2ad
https://humanwhocodes.com/blog/2019/01/stop-using-default-exports-javascript-module/
https://rajeshnaroth.medium.com/avoid-es6-default-exports-a24142978a7a
With named exports, one can have multiple named exports per file. Then import the specific exports they want surrounded in braces. The name of imported module has to be the same as the name of the exported module.
// imports
// ex. importing a single named export
import { MyComponent } from "./MyComponent";
// ex. importing multiple named exports
import { MyComponent, MyComponent2 } from "./MyComponent";
// ex. giving a named import a different name by using "as":
import { MyComponent2 as MyNewComponent } from "./MyComponent";
// exports from ./MyComponent.js file
export const MyComponent = () => {}
export const MyComponent2 = () => {}
You can also alias named imports, assign a new name to a named export as you import it, allowing you to resolve naming collisions, or give the export a more informative name.
import MyComponent as MainComponent from "./MyComponent";
You can also Import all the named exports onto an object:
import * as MainComponents from "./MyComponent";
// use MainComponents.MyComponent and MainComponents.MyComponent2 here
One can have only one default export per file. When we import we have to specify a name and import like:
// import
import MyDefaultComponent from "./MyDefaultExport";
// export
const MyComponent = () => {}
export default MyComponent;
The naming of import is completely independent in default export and we can use any name we like.
From MDN:
Named exports are useful to export several values. During the import, one will be able to use the same name to refer to the corresponding value.
Concerning the default export, there is only a single default export per module. A default export can be a function, a class, an object or anything else. This value is to be considered as the “main” exported value since it will be the simplest to import.
There aren't any definitive rules, but there are some conventions that people use to make it easier to structure or share code.
When there is only one export in the entire file, there is no reason to make it named.
Also, when your module has one main purpose, it could make sense to make that your default export. In those cases you can extra named exports
In react for example, React is the default export, since that is often the only part that you need. You don't always Component, so that's a named export that you can import when needed.
import React, {Component} from 'react';
In the other cases where one module has multiple equal (or mostly equal) exports, it's better to use named exports
import { blue, red, green } from 'colors';
1st Method:-
export foo; //so that this can be used in other file
import {foo} from 'abc'; //importing data/fun from module
2nd Method:-
export default foo; //used in one file
import foo from 'blah'; //importing data/fun from module
3rd Method:-
export = foo;
import * as foo from 'blah';
The above methods roughly compile to the following syntax below:-
//all export methods
exports.foo = foo; //1st method
exports['default'] = foo; //2nd method
module.exports = foo; //3rd method
//all import methods
var foo = require('abc').foo; //1st method
var foo = require('abc')['default']; //2nd method
var foo = require('abc'); //3rd method
For more information, visit to Default keyword explaination
Note:- There can be only one export default in one file.
So whenever we are exporting only 1 function, then it's better to use default keyword while exporting
EASIEST DEFINITION TO CLEAR CONFUSIONS
Let us understand the export methods, first, so that we can analyze ourselves when to use what, or why do we do what we do.
Named exports: One or more exports per module. When there are more than one exports in a module, each named export must be restructured while importing. Since there could be either export in the same module and the compiler will not know which one is required unless we mention it.
//Named export , exporting:
export const xyz = () =>{
}
// while importing this
import {xyx} from 'path'
or
const {xyz} = require(path)
The braces are just restructuring the export object.
On the other hand , default exports are only one export per module , so they are pretty plain.
//exporting default
const xyz =() >{
};
export default xyz
//Importing
import xyz from 'path'
or
const xyz = require(path)
I hope this was pretty simple to understand, and by now you can understand why you import React modules within braces...
Named Export: (export)
With named exports, one can have multiple named exports per file. Then import the specific exports they want surrounded in braces. The name of imported module has to be the same as the name of the exported module.
// imports
// ex. importing a single named export
import { MyComponent } from "./MyComponent";
// ex. importing multiple named exports
import { MyComponent, MyComponent2 } from "./MyComponent";
// ex. giving a named import a different name by using "as":
import { MyComponent2 as MyNewComponent } from "./MyComponent";
// exports from ./MyComponent.js file
export const MyComponent = () => {}
export const MyComponent2 = () => {}
Import all the named exports onto an object:
// use MainComponents.MyComponent and MainComponents.MyComponent2 here
import * as MainComponents from "./MyComponent";
Default Export: (export default)
One can have only one default export per file. When we import we have to specify a name and import like:
// import
import MyDefaultComponent from "./MyDefaultExport";
// export
const MyComponent = () => {}
export default MyComponent;
Note: The naming of import is completely independent in default export and we can use any name we like.
Here's a great answer that explains default and named imports in ES6
I'm new to ES6 and a bit confused with the way classes are exported and imported. It seems many different notations are valid but work differently.
I wrote a class like this in src/web-api.js:
class WebApi {
// ...
}
export { WebApi };
Which I import with:
import { WebApi } from './src/web-api.js'
This works fine, but before I've tried the same thing without curly brackets and it didn't work:
export WebApi; // Tells me '{' expected
import WebApi from './src/web-api.js'; // No syntax error but WebApi is undefined
Even though on the MDN documentation for export, the notation export expression; appears to be valid.
Likewise, this is how React is imported in my application file:
import React, { Component } from 'react';
Why is one class with and another one without curly brackets? In general, how can I tell when to use and not to use curly brackets?
ES6 offers many ways to manage modules through import/export. But there are basically two main strategies:
Default export/import with export default and import module from './module'
Multiple exports/imports with export and import {member} from './module' or import * as module from './module'
(Mixing both is possible but not recommended.)
Module to export/import
function foo() {
console.log('Foo');
}
function bar() {
console.log('Bar');
}
Strategy #1: Default export/import
Export (module.js)
function foo() {
console.log('Foo');
}
function bar() {
console.log('Bar');
}
export default {foo, bar};
/*
{foo, bar} is just an ES6 object literal that could be written like so:
export default {
foo: foo,
bar: bar
};
It is the legacy of the "Revealing Module pattern"...
*/
Import (main.js)
import module from './module';
module.foo(); // Foo
module.bar(); // Bar
Strategy #2: Multiple exports/imports
Export (module.js)
export function foo() {
console.log('Foo');
}
export function bar() {
console.log('Bar');
}
Import (main.js)
import {foo, bar} from './module';
foo(); // Foo
bar(); // Bar
/*
This is valid too:
import * as module from './module';
module.foo(); // Foo
module.bar(); // Bar
*/
As I said previously, ES6 modules are much more complex than that. For further information, I recommend you to read Exploring ES6 by Dr. Axel Rauschmayer, especially this chapter: http://exploringjs.com/es6/ch_modules.html.
In your case, if you import from the src/web-api.js file without the curly braces, you should have anexport default something in the src/webfile-api.js
Without curly braces
class WebApi {...};
export default WebApi;
In your file
import WebApi from './src/web-api.js'
// Here, the element withexport default in the src/web-api.js file should be imported without the curly braces anywhere.
PS: It must have only one export default for each file.
With curly braces
export { WebApi }
In your file
import {WebApi} from './src/web-api.js'
Dan Abramov explains clearly the export/import methods in ES6 at this answer.
When should I use curly braces for ES6 import?
The braces are just syntactic sugar. It will use the variable name as the object key, for example:
const a = 1;
const test = {a}; // same as {a: 1};
It can also be used to destructure the object by its variable name. It will check if the object has any properties with the same value as the variable name and then output a value if one is found:
const test = {a: 1};
const {a} = test; // a = 1
In modules, the general use case is that when you import there is usually braces since modules get imported as MODULE.function or MODULE.class. It'd be more intuitive to look at exports first:
For exporting, it's using the syntactic sugar I mentioned before - you're exporting it as an object. When you do export { WebApi }; what you're really doing is export {WebApi: WebApi}. This makes it easier to access things as you can just do 'MODULE.WebApi' now to access the class instead of having it pollute the namespace unnecessarily. This is also required for all exports!
Moving on to imports, what you're doing in the import statements is essentially destructuring the module object and picking a property to save into a variable of the same name. In your case, when you do import {WebApi} from './src/web-api.js' you'd be doing something like import WebApi = web-api.js['WebApi'] from './src/web-api.js' (this isn't valid syntax but just to show you what it's doing in the background). This is also required to properly import module functions/classes. There is also the option of importing the whole module, just as NodeJS does: import * as ModuleName from './src/module.js'. This will put all of exported functions/classes into the ModuleName object so that it can be treated as a normal module.
However, if a module has a default export, braces are not not needed for import and export. For example, react probably has export default React in its files - that's why there doesn't need to be braces around it when you do import React from 'react'
Hope I wasn't too confusing and let me know if I can clarify anything!