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alangpierce committed a4a596e May 18, 2020
Fixes #536

We already completely remove uninitialized class fields for now, which agrees
with the `declare` behavior, so the only change needed here was to parse the
syntax without crashing. In the future, when we remove the class fields
transform, we'll need to distinguish declare vs non-declare fields.


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May 17, 2020


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Sucrase is an alternative to Babel that allows super-fast development builds. Instead of compiling a large range of JS features to be able to work in Internet Explorer, Sucrase assumes that you're developing with a recent browser or recent Node.js version, so it focuses on compiling non-standard language extensions: JSX, TypeScript, and Flow. Because of this smaller scope, Sucrase can get away with an architecture that is much more performant but less extensible and maintainable. Sucrase's parser is forked from Babel's parser (so Sucrase is indebted to Babel and wouldn't be possible without it) and trims it down to a focused subset of what Babel solves. If it fits your use case, hopefully Sucrase can speed up your development experience!

Sucrase has been extensively tested. It can successfully build the Benchling frontend code, Babel, React, TSLint, Apollo client, and decaffeinate with all tests passing, about 1 million lines of code total.

Sucrase is about 20x faster than Babel. Here's one measurement of how Sucrase compares with other tools on a large TypeScript codebase with 4045 files and 661081 lines of code:

             Time      Speed
Sucrase      2.928s    225752 lines per second
swc          13.782s   47966 lines per second
TypeScript   39.603s   16693 lines per second
Babel        52.598s   12569 lines per second


The main configuration option in Sucrase is an array of transform names. These transforms are available:

  • jsx: Transforms JSX syntax to React.createElement, e.g. <div a={b} /> becomes React.createElement('div', {a: b}). Behaves like Babel 7's React preset, including adding createReactClass display names and JSX context information.
  • typescript: Compiles TypeScript code to JavaScript, removing type annotations and handling features like enums. Does not check types. Sucrase transforms each file independently, so you should enable the isolatedModules TypeScript flag so that the typechecker will disallow the few features like const enums that need cross-file compilation.
  • flow: Removes Flow type annotations. Does not check types.
  • imports: Transforms ES Modules (import/export) to CommonJS (require/module.exports) using the same approach as Babel and TypeScript with --esModuleInterop. Also includes dynamic import.
  • react-hot-loader: Performs the equivalent of the react-hot-loader/babel transform in the react-hot-loader project. This enables advanced hot reloading use cases such as editing of bound methods.

These proposed JS features are built-in and always transformed:

Unsupported syntax

All JS syntax not mentioned above will "pass through" and needs to be supported by your JS runtime. For example:

  • Decorators, private fields, throw expressions, generator arrow functions, and do expressions are all unsupported in browsers and Node (as of this writing), and Sucrase doesn't make an attempt to transpile them.
  • Object rest/spread, async functions, and async iterators are all recent features that should work fine, but might cause issues if you use older versions of tools like webpack. BigInt and newer regex features may or may not work, based on your tooling.

JSX Options

Like Babel, Sucrase compiles JSX to React functions by default, but can be configured for any JSX use case.

  • jsxPragma: Element creation function, defaults to React.createElement.
  • jsxFragmentPragma: Fragment component, defaults to React.Fragment.

Legacy CommonJS interop

Two legacy modes can be used with the import transform:

  • enableLegacyTypeScriptModuleInterop: Use the default TypeScript approach to CommonJS interop instead of assuming that TypeScript's --esModuleInterop flag is enabled. For example, if a CJS module exports a function, legacy TypeScript interop requires you to write import * as add from './add';, while Babel, Webpack, Node.js, and TypeScript with --esModuleInterop require you to write import add from './add';. As mentioned in the docs, the TypeScript team recommends you always use --esModuleInterop.
  • enableLegacyBabel5ModuleInterop: Use the Babel 5 approach to CommonJS interop, so that you can run require('./MyModule') instead of require('./MyModule').default. Analogous to babel-plugin-add-module-exports.



yarn add --dev sucrase  # Or npm install --save-dev sucrase

Often, you'll want to use one of the build tool integrations: Webpack, Gulp, Jest, Rollup, Broccoli.

Compile on-the-fly via a require hook with some reasonable defaults:

// Register just one extension.
// Or register all at once.

Compile on-the-fly via a drop-in replacement for node:

sucrase-node index.ts

Run on a directory:

sucrase ./srcDir -d ./outDir --transforms typescript,imports

Call from JS directly:

import {transform} from "sucrase";
const compiledCode = transform(code, {transforms: ["typescript", "imports"]}).code;

What Sucrase is not

Sucrase is intended to be useful for the most common cases, but it does not aim to have nearly the scope and versatility of Babel. Some specific examples:

  • Sucrase does not check your code for errors. Sucrase's contract is that if you give it valid code, it will produce valid JS code. If you give it invalid code, it might produce invalid code, it might produce valid code, or it might give an error. Always use Sucrase with a linter or typechecker, which is more suited for error-checking.
  • Sucrase is not pluginizable. With the current architecture, transforms need to be explicitly written to cooperate with each other, so each additional transform takes significant extra work.
  • Sucrase is not good for prototyping language extensions and upcoming language features. Its faster architecture makes new transforms more difficult to write and more fragile.
  • Sucrase will never produce code for old browsers like IE. Compiling code down to ES5 is much more complicated than any transformation that Sucrase needs to do.
  • Sucrase is hesitant to implement upcoming JS features, although some of them make sense to implement for pragmatic reasons. Its main focus is on language extensions (JSX, TypeScript, Flow) that will never be supported by JS runtimes.
  • Like Babel, Sucrase is not a typechecker, and must process each file in isolation. For example, TypeScript const enums are treated as regular enums rather than inlining across files.
  • You should think carefully before using Sucrase in production. Sucrase is mostly beneficial in development, and in many cases, Babel or tsc will be more suitable for production builds.

See the Project Vision document for more details on the philosophy behind Sucrase.


As JavaScript implementations mature, it becomes more and more reasonable to disable Babel transforms, especially in development when you know that you're targeting a modern runtime. You might hope that you could simplify and speed up the build step by eventually disabling Babel entirely, but this isn't possible if you're using a non-standard language extension like JSX, TypeScript, or Flow. Unfortunately, disabling most transforms in Babel doesn't speed it up as much as you might expect. To understand, let's take a look at how Babel works:

  1. Tokenize the input source code into a token stream.
  2. Parse the token stream into an AST.
  3. Walk the AST to compute the scope information for each variable.
  4. Apply all transform plugins in a single traversal, resulting in a new AST.
  5. Print the resulting AST.

Only step 4 gets faster when disabling plugins, so there's always a fixed cost to running Babel regardless of how many transforms are enabled.

Sucrase bypasses most of these steps, and works like this:

  1. Tokenize the input source code into a token stream using a trimmed-down fork of the Babel parser. This fork does not produce a full AST, but still produces meaningful token metadata specifically designed for the later transforms.
  2. Scan through the tokens, computing preliminary information like all imported/exported names.
  3. Run the transform by doing a pass through the tokens and performing a number of careful find-and-replace operations, like replacing <Foo with React.createElement(Foo.

Because Sucrase works on a lower level and uses a custom parser for its use case, it is much faster than Babel.


Contributions are welcome, whether they be bug reports, PRs, docs, tests, or anything else! Please take a look through the Contributing Guide to learn how to get started.

License and attribution

Sucrase is MIT-licensed. A large part of Sucrase is based on a fork of the Babel parser, which is also MIT-licensed.

Why the name?

Sucrase is an enzyme that processes sugar. Get it?


Super-fast alternative to Babel for when you can target modern JS runtimes




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