But again, it feels like export default identifier should have been special-cased in the same way for consistency. Taking: It logs "changed", because export default function is given its own special semantics; the function is passed by reference in this case.
an index.js file comes in. The above fails, because hello in module.js points to the hidden variable exported by main.js, and it's accessed before it's initialized.
You could then import the re-exported members from the same module. So every page died because I was trying to use a higher-order component to configure SWR, but in fact, it was just the whole exported module. generate preliminary obsolete So: Unlike export default thing, export { thing as default } exports thing as a live reference. Normally React components are default exports, and other things such as PropTypes and functions / hooks are exported as named exports. work, the spec gives export default thing different semantics to export thing. The bit after export default is treated like an expression, which allows for things like export default 'hello!' In the end, the problem was simply my mis-import of the SWRConfig in my _app.js Next.JS page layout template component. All content is copyright protected. explicitly specify the name index when importing (in some environments).
However, we can still change the names of named exports/imports by using aliases with as. Since you can't use export {} to export values directly, it always passes a live reference. The pattern you often see is - re-export members of different files from a file But your explanation has made things much clearer. All rights are expressly reserved. The SWR team didnt do that in this case.
Also, doesnt the restriction of only sending one object with the default export become irrelevant if we just export everything as separate named exports?
As a library creator, I will try to make sure to stick with the components-as-default-exports mantra wherever possible. This came to light when Dominic messaged me about circular dependencies. confuse maintainers. @mtf? During the import, it is mandatory to use the same name of the corresponding object.But a default export can be imported with any name, //logs TEST to the console, uses the private uppercaseName function even though we dont have access to uppercaseName through our imported Animal module and cannot call it explicitly.
However, this only works due to hoisting, which lifts both function definitions above both of their calls. If we change module.js to: it no longer matches the special case, so it logs thing() {}, as it's passed by value again. But, as with my last few posts, please don't add this to your interview questions, just avoid circular dependencies .
homebuyers supposed useless thepennyhoarder actuality These are similar to the old node-style module.exports syntax, essentially in that you can only export one thing.
We combine named exports and default exports when we want to give a module the ability to define the default export of the module and other, potentially useful, pieces of a module (the named exports).
But in this library, the hook was the default component and the component was a non-default export. file called index.js. In general it's better to be explicit and import from './utils/index.js', After some testing, discussion, and *ahem* chatting to the V8 team, we figured it out, but I learned something new about JavaScript along the way.
// default and named imports (all from index.js). I suspect this is another reason export default function was special-cased; to make hoisting work as expected. Dominic Elm DM'd me on Twitter to ask me questions about circular dependencies, and, well, I didn't know the answer. I learned something new. The syntax for re-exporting members of another module is: The two lines from the example above can be combined into a single line if But I digress. excel text autotext export insert easily entry create dialog button box
For example, with: This works!
I am using this one in my own projects on classes. Here is how you would re-export the exported members of another-file.js from a Named exports are just what they sound like you export them by name, and can have any number of exports in the same file: Now, we can also have default exports.
import {MyClass} from './moduleToExport.js'; Anything else may So there you go! n.b. 
This means when a different value is assigned to thing in module.js, that change is reflected in the import in main.js. He said that some earlier designs of default exports were in the form export default = thing, which would have made it more obvious that thing is treated as an expression. shapefile It's to do with how these statements change behaviour when they're expressions: function and class statements create an identifier in the scope/block, whereas function and class expressions do not (although their names can be used internal to the function/class). 
It's as if it's assigned to a hidden variable before it's exported, and as such, when thing is assigned a new value in the setTimeout, that change isn't reflected in the hidden variable that's actually exported. First we need to talk about 'hoisting': You might have encountered the age-old weird thing JavaScript does to functions: Function definitions are essentially moved to the top of the file.
At least in terms of ease. First up: In the above example, thing is the same as thing in ./module.js. @aubsrey? Imports are 'live bindings' or what some other languages call a 'reference'. Circular dependencies are allowed in JavaScript, but they're messy and should be avoided. There are two different types of export, named and default.
The destructuring behaviour isn't unique to imports: The above feels natural in my opinion. By convention, a file that exports only one class, function, or constant should be named for that class, function or constant.
Huge thanks to Toon Verwaest, Marja Hltt, and Mathias Bynens from the V8 team for making sure I'm using the correct terminology throughout this post, Dave Herman and Daniel Ehrenberg for giving me some of the history around this, proof-readers Surma, Adam Argyle, Ada Rose Cannon, Remy Sharp, Lea Verou (heh, I got a lot of folks to read this, I wanted it to make as much sense as possible) and of course thanks to Dominic Elm for triggering this whole adventure!
import MyClass from './moduleToExport.js'; is more obvious and direct. For example, in Node.js, the default mode is explicit, which means, you have to import from. Re-export values from another file in JavaScript, // import (only if you need to use them in index.js). Also from the documentation, the default export can be imported with ANY name. rather than ./utils, because this syntax is supported in more environments and I was using Next.js its got a page templating system, and this error lived there. name exports as export {myFunction, myConstant} from './another-file.js and You can have multiple named exports per module but only one default export[]Named exports are useful to export several values. I guess it's too late to change it now. Tags: javascript, lessons learned, modules If I understand it correctly, there is no difference between the cases. In ECMAScript JavaScript module syntax (import / export), exported elements can either be named, or you can define a single default export. It didnt answer the question right. the default export as export {default} from './another-file.js'. Technically this is not a bug, it is just sloppy coding. So: I said that the bit after export default is treated like an expression, but there are exceptions to that rule.
The example above directly re-exports the 2 named exports and the default seem completely the same in my experiences.
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However, we do have access to the Animal class, which in turn does have access and uses the uppercaseName. So the fact still remains that theres really no moving reason to use default export with named exports other than for geeking out? The destructured import doesn't pick up the change because destructuring assigns the current value (rather than a live reference) to a new identifier. Also from the documentation, the default export can be imported with ANY name. Here is an example of a file that has 2 named exports and a default export.
I agree!
In THIS post you said that we would not have access to variables that were not imported. Is there realy a diffrence between these 2 ways of exporting/importing ? Feel free to throw me an email, unless you're a recruiter, or someone trying to offer me 'sponsored content' for this site, in which case write your request on a piece of paper, and fling it out the window. I'm going to leave the circular dependency stuff to the end of the article, as it isn't totally related. Well, its nice to have multiple options and a complete overview about exporting / importing but I dont see any benefit by combining different exports methods. I did it a few times in the past and it caused me pain, The problem was not surfaced by a linter check or a compiler, because JS doesnt care what you call your import with default objects. Case, dots ( .
To re-export values from another file in JavaScript, make sure to export the If youd try calling uppercaseName in main.js youd get an error, same as is explained in the other case youve referred to. This kinda makes export default identifier the odd one out. export default MyClass;
export. Special-casing functions also helps with circular dependencies, but I'll get onto that shortly. It isn't just export default function export default class is special-cased in the same way. database mysql export godaddy help file exported field check leave custom name been extracted into separated files, and you might not want to have 5 lines of called index.js. I spent the next four hours trying to debug this component, which used the properties referenced in the API (mutate, data, error, isValidating): Mainly I focused on the useSWR hook, because it used those three variables that were referenced in the stack. export { variable1, variable2, myObject } module.js executes first, and as a result it tries to access hello before it's instantiated, and throws an error. For example, these other pieces may be the internals of a private variable where we have access to part of a private variable, but not the entire private variable. The same is true with other functions, primitive data types (even theyre objects too).
and I wasn't expecting those to be "initial"! This is the main advantage of re-exporting values - you can re-export the members of multiple files from a single module, which would simplify your imports.
Category: tips and tricks, This is one of those blog posts I write to save others from the pain I suffer.
which is something you can't do with named exports: To make export default 'hello!' Note the distinction between we have access, and 'the object has access. Next.js gave me a rather confusing stack trace about this: Its not too confusing now that I figured out what I was doing wrong, but I had thought I imported the React component SWRConfig when in fact I imported the whole library as SWRConfig.
Powered by Discourse, best viewed with JavaScript enabled, FAQ: Intermediate JavaScript Modules - Combining Export Statements.
If you are re-exporting members from multiple files, you would have to use a line of code for each file. React developers typically (but not always) export components as the default Keep that in mind. I get that export default 'hello!'
The above logs undefined, because the declaration of var foo in the function is hoisted to the start of the function, but the assignment of 'hello' is left where it is. needs to be passed by value, but since there's a special case that makes export default function passed by reference, it feels like there should be a special case for export default identifier too. becomes transparent background management python I had a chat with Dave Herman about this, who was involved in the design of JavaScript modules. transcriber step Setup is effortless and analysis is automatic for most languages, Fast, accurate analysis; enterprise scalability. That only really happens with plain function declarations: If you try to access a let/const/class identifier before it's instantiated, it throws an error. What is the difference between these situations?
It logs "hello" then "foo". I think this is also similar to what @biirra was getting at in her post above: Can anyone shed any light on this?
Why would I want to combine named exports with default exports? If we change the code to: it fails.
Hello, Im Jake and that is my tired face. Did you spot the bug? If main.js is changed to use export default function hello(), again it doesn't fail, but this time it's because it hits that super-magic-special-case of export default function. // These give you a live reference to the exported thing(s): // This assigns the current value of the export to a new identifier. export default MyClass;
the way to do it if you can is to just use a plain function, JavaScript is so picky and odd. However, this pattern is implicit and sometimes frowned upon. If main.js is changed to use export { hello as default }, it doesn't fail, because it's passing the function by reference and gets hoisted. So tuck this away in your head whenever you get a strange Objects are not valid as a React child to make sure youve actually imported a true React component, and not the wrong object. Whichever option (default, or named) that the object was imported, it will still have the same access to the function only found in the other file.
The values Im still confused about why we would want to use export default myObject at all, instead of just adding it along with our other named exports e.g.
2008-2022 SonarSource S.A., Switzerland. Ok then I guess this means aubsrey wasnt really on point with her example. In the case addressed in this thread, we dont have access to anything that wasnt imported, namely - uppercaseName. While I have a soapbox, might I complain about having to do this everywhere I want to export a default that isnt a straightforward function?
import MyClass from './moduleToExport.js'; I am wondering. Also Next.js does this for data fetching exporting the component as default, and the data fetching API methods as non-default exports.
The way I understand it, the default export is to send only one statement. The potential gotcha here is that named static imports (import { thing } ) kinda look like destructuring, but they don't behave like destructuring. Come on, Ken, SWRConfig is NOT the default export of swr.
I was confused like @edgarmolas about this same thing. But useSWR IS the default export of the swr module, so that code was fine. you're re-exporting members of the same file. ), underscores ( _ ) and dashes (-) are ignored from the name comparison. Yeh - I second that thanks! imports just for utility functions or constants - this is when re-exporting from
I know that maybe sounds obvious, but what about: In this case module.thing is the same as thing in ./module.js, whereas destructuredThing is a new identifier that's assigned the value of thing in ./module.js, and that behaves differently.
Postfixes in Can a named export be an object with methods.
exported elements can either be named, or you can define a single default export, exporting the component as default, and the data fetching API methods as non-default exports, Unlike many other React libraries, SWR exports a, I will run into this again.
This was seen as a bit of a gotcha, which is why let/const/class throw an error in similar cases.
In a practical way, I believe that means whatever object you want to directly call in your file, must be imported, but you dont need to worry about importing each method, function or variable that object uses (assuming, of course, they are properly defined within the same scope). The name index.js is important because you don't have to In preparing a demo for an upcoming talk, I made this mistake using the Vercel SWR library. March 17, 2022March 8, 2022 by Ken Rimple and export default 1 + 2. What if you accidentally tried to import what you thought was a named export but left off the braces? A default export can be imported with any variable name: So, its easy to make a mistake and misname the exported component something different. Many of the files you use might make use of multiple utility functions that have and This also 'works' for export default thing, but since thing is treated as an expression it causes thing to be passed by value. This is the example Dominic gave me. Im a developer advocate for Google Chrome. export {MyClass}; IDE extension that lets you fix coding issues before they exist!
But you get only ONE.
I cant see any real benefit of the default export, other than being able to import it using a different name, as @jellotimez explains in his post above: The way I understand it, the default export is to send only one statement. I did not understand the default exports.If someone can help me here. If export default function wasn't special-cased, then the function would be treated as an expression, and the log would be "undefined".
That thing can be a complex object or just a simple primitive. Herein lies the beginning of my pain. I removed the entire API call, and still I got the same error.
can be imported from the file that re-exported them.
I got confused after reading that. filenames like .dev in my.class.dev.js are also ignored.
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