is a syntax formatting tool for JavaScript. It's been widely adopted and used in major projects like React and Jest, and we have over 10,000 stars on github in about 4 months. It’s been tremendous to watch it grow. I’m going to talk about why I built it and how it can help you. * A question I like to ask: "what are we wasting our time on?" * We waste so much time when we build software. Some of it is social problems and team dynamics. Some of it is technical problems, because building software is difficult.
Design • Tooling (CI/reviews/etc) • Deployment • Monitoring/reporting/logging • Security • Accessibility • User experience • And more! * There are so many things to get right… * You have to prioritize, and do it wisely. If you spend time optimizing a bunch of code and making it more accessible, only to find that you need to heavily refactor your code later due to new needs, you've completely wasted your time. Same with designing something to find out you don't need it. You'll never get rid of waste, but good scheduling will help reduce it. * All this work is "large scale", and I want to talk about "small scale" work…
all the stuff that you do every few minutes. Typing code, getting feedback from your editor, refactoring a function into multiple smaller functions, etc. Optimizing this kind of work is about streamlining your environment. Making it as easy as possible to get the ideas from your head into working code. * You end up wasting time here with small distractions. Anything that keeps bringing you out of focus makes you slower. It's a huge difference if your build step takes 1 second or 10 seconds. If you're tools error frequently. If your API are bad and you keep having to look up documentation every time you need to call one of them. The problem isn't the actual time wasted, but the context switching and distractions which prevent you from being able to focus.
or known as hot module replacement. This establishes a direct connection from your brain to the screen, allowing you to make any changes and instantly see them without having to reload and fiddle with the UI. It only saves a few seconds, but the effect is quite transformative when you're experimenting with a lot of changes. * I believe there is another place where we are constantly distracted when writing code, and it's slowing us down: syntax. It’s slow to write code that involves complex syntax, and think about how to format it, especially when continually refactoring code. It distracts you and wastes time.
syntax. Languages like Lisp and Scheme do this by using s-expressions, where the entire language is made up of parentheses and brackets. This allows powerful tools like paredit and parinfer, which performs structural editing of code. Work with expressions, not lines of code. (live demo, open up clojure.cljs, search for update-nl-state) But what are we to do in languages like JavaScript? How can we narrow this gap between thinking of code and writing it, when complex syntax makes it slow to write and refactor code?
semantic errors and pointing out pitfalls in your code, but it's not good for formatting because it's very piecemeal and only forces silly stuff like whitespace in the right place. We need a more holistic approach for formatting. Last fall started writing ReasonML code, and they already have a tool out-of-the-box for formatting it. As a beginner it was enlightening to write some code, try to format it, it would tell me what was invalid, I'd fix, and then it would format it in standard ReasonML code automatically. This opened my eyes, and I never wanted to manually format code again. (have gif)
wanted a tool that would cast aside any of the original formatting and completely reformat a file. This would not only let you write whatever kind of ugly code you want, but also force consistency across teams.
do this is to compile JavaScript to an AST and pretty-print the AST. There wasn't any tool that already did this good enough. Recast was the best at the time, but it has different goals like respecting the original formatting, so I forked it and had a good place to start. I built it and it works! Let me show you an example. (live demo) * open ReactChildFiber.js and search for useFiber, add conditional and function call with object as last param * open budget.js and move JSX around
printing" and there's already a large amount of research on this over the last few decades. In my research I found Wadler's paper, "A Prettier Printer", which describes an algorithm for doing this. This is the algorithm that prettier still uses today. You might be wondering: why is this so hard? Can't you just apply a few fixed rules when printing each AST node type? No. Let me show you an example
}}; Our style enforces newlines at the beginning and end of function bodies, so it would do this which fixes that problem But that's clearly not an ideal format…
flag: false, count: 5 })); But all of this is done only because the `makeErrorMessage` function was added. Adding any other property that doesn't have any newlines shouldn't break up anything There are *all sorts* of cases like this where we want to do a specific thing based on how syntax is composed.
nothing that it's a much simpler printer that *does* mostly implement fixed rules for each type of syntax. The reason they can get away with it is because Go's syntax isn't as complex as JavaScript, and because they've had that tool since Go came out, so they were able to enforce style from the beginning. JavaScript's syntax is a lot more complex, and how we use it is a lot more diverse. We do a lot of weird things in JS. For this tool to be successful, we need to accommodate all of this so people will actually use it. There's something important that I haven't talked about yet, and it's a key part of all of this: the maximum line length. This is a really important factor if you want this to work. Take `gofmt` for example, it allows this style
int, arg6 int, arg7 int, arg8 int) (res int) { return 1 } That line is 100 characters long. gofmt doesn't ever enforce a maximum line length. It's fine for Go because the syntax and general patterns already encourage spreading out code vertically, and the syntax is simple enough that if something really gets too long, it's not hard to break it up in a consistent way. But JavaScript is more complex and there is a lot of diverse styles already, and it's too easy to write inconsistent code because we break it up differently in different places
callFunction2(arg1, arg2); const variable = someCondition && otherCondition ? callFunction1(arg1, arg2) : callFunction2(arg1, arg2); const variable = someCondition && otherCondition ? callFunction1(arg1, arg2) : callFunction2(arg1, arg2); Every time I need to break up code I need to decide what the format should be, and it's usually a unique composition of syntax that takes a good bit of energy to think about the best way to break that specific piece of code up. This brings us out of focus. It wastes time. It's also very likely that your coworkers would break it up differently, causing style inconsistency. The maximum line length is a critical piece of information for formatting your code. That's what really makes prettier different, and capable of taking on the problem of JavaScript formatting. Without it we can't actually enforce a consistent style and you would still be required to intervene all the time. We want you to be able to trust prettier completely and not think about formatting at all.
that takes a string of code and a maximum line length, and returns a formatted string of code. That's all we need. `refmt`, the formatter for ReasonML, works this way too. It works by trying to fit as much code as it can within the width parameter, and breaking expressions apart if it can't fit. I'll show you an example. DEMO: Here is the entire code of React, and right now the print width is set to 80. It's breaking up the code to all fit within 80. Here I have a special build of prettier that aggressively inlines code. This build never forces a newline, meaning it will fit the entire file into a single line of code if it can. The normal build of prettier forces hard newlines in many places and wouldn't do this. (live demo: show React file, bump up print width to a high number, then to a really small number)
parse the JavaScript using any of the currently available parsers. Then we take this AST and generate a "document" representation of it, which is a very simple language that describes groups of strings. Then we generate a string from the document representation given a line width. The reason there's a sort of "intermediate" representation is because we need to measure the strings in order to see if they will fit on the same line. This has to be a second pass where all the strings are available, otherwise it'd be a lot more complicated as we'd have to measure *after* the real code is generated and do a lot of reprinting. It's actually a lot more performant to emit a basic document representation that we can measure really fast and make a lot of decisions there.
that tells the printer that it can break everything inside of it across lines. If you think about it, when the printer measures an entire program and it doesn't fit on one line, how does it know what to break? Without `group` it would either print the whole program on one line or, when it doesn't fit, break the entire program up. `group` limits the breaking to just that section.
"reallyLongFunction(", line, "longArgument1,", line, "longArgument2,", line, "longArgument3", line, ")" ])) Here is the document representation of them (somewhat simplified). `group`, `concat`, and `line` are all “commands” that instruct the printer. Here we have 2 groups that simply combine a bunch of strings, with newlines in between them if they don’t fit. The printer will measure each group separately and break any of them that doesn’t fit.
second group, since it’s a lot longer, is going to break first before the first group. Groups give the printer an atomic unit to measure and break if it doesn’t fit.
"arg3", line "arg4", line, ")" ])), line, ")" ])) The document representation looks like this. There are two groups, and one is inside the other. Prettier will always break the outer group first. This makes sense if you think about it: the outer group always encloses the inner group, so it's always going to hit the max line length first.
prettier operates, but making a real-world JavaScript formatter is actually a lot harder. There are so many diverse patterns of JS code that naively applying a formatter to real code would make a lot of stuff ugly. So we have all kinds of hand-tuned formats embedded in the prettier logic. Here are some examples
foremost win with this tool is consistency. Not necessarily consistency across the entire JS community, but at least consistency across a company or at least a team. The second win is teachability. Prettier gives a certain tactile feeling to your code, allowing you to actually learn from what it does. The third, and most important, win is freedom. Prettier is about a lot more than just consistency. You just don’t have to worry about how your code looks anymore. This is immensely freeing when writing code, and allows you to focus on the real problem and move around code quickly. There’s an interesting side effect of all of this, which is that if prettier gets used on a majority of projects, we have a chance to allow you, as an individual, to view the code however you want. You can load the code, print it with a custom printer yourself, and you just need to re-print it with the project before committing. You'd be able to always view the code in your own style no matter where it came from. This is why we've relaxed our opinion on options, and prettier comes with several options for major things like semicolons and tabs. You could even write a completely custom printer that prints it in a totally different syntax. For what it's worth, I think there's great value in standardizing on a general format for JavaScript. But the fact is that the JavaScript community is *massive* and I don't think we'll ever converge on a single style for all. It's possible that given the flexibility to view code how you want, the JavaScript stored on the disk becomes an artifact that you don't really care about and we never have to debate about syntax again. The community has been phenomenal and we’ve received so many great contributions….
announce that we just released a new version that includes TypeScript and CSS support. We already have really good JSX and Flow support, and it’s great that we can support other variants as well. I want to thank Christopher Chedeux specifically for being such an active maintainer and contributor, and pushing me to complete this project. Thank you.
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