Sucrase






Try it out

Quick usage

```bash

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

node -r sucrase/register main.ts

```

Using the ts-node integration:

```bash

yarn add --dev sucrase ts-node typescript

./node_modules/.bin/ts-node --transpiler sucrase/ts-node-plugin main.ts

```

Project overview

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 when compiling the Jest codebase 3 times,

about 360k lines of code total:

```text

Time Speed

Sucrase 0.57 seconds 636975 lines per second

swc 1.19 seconds 304526 lines per second

esbuild 1.45 seconds 248692 lines per second

TypeScript 8.98 seconds 40240 lines per second

Babel 9.18 seconds 39366 lines per second

```

Details: Measured on July 2022. Tools run in single-threaded mode without warm-up. See the

benchmark code

for methodology and caveats.

Transforms

The main configuration option in Sucrase is an array of transform names. These

transforms are available:

• jsx: Enables JSX syntax. By default, JSX is transformed to React.createClass,

but may be preserved or transformed to _jsx() by setting the jsxRuntime option.

Also adds 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. The Sucrase option keepUnusedImports

can be used to disable all automatic removal of imports and exports, analogous to TS

verbatimModuleSyntax.

• 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. If preserveDynamicImport is specified in the Sucrase

options, then dynamic import expressions are left alone, which is particularly

useful in Node to load ESM-only libraries. If preserveDynamicImport is not

specified, import expressions are transformed into a promise-wrapped call to

require.

• 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.

• jest: Hoist desired jest method calls above imports in

the same way as babel-plugin-jest-hoist.

Does not validate the arguments passed to jest.mock, but the same rules still apply.

When the imports transform is *not* specified (i.e. when targeting ESM), the

injectCreateRequireForImportRequire option can be specified to transform TS

import foo = require("foo"); in a way that matches the

TypeScript 4.7 behavior

with module: nodenext.

These newer JS features are transformed by default:

• Optional chaining: a?.b
• Nullish coalescing: a ?? b
• Class fields: class C { x = 1; }.

This includes static fields but not the #x private field syntax.

• Numeric separators:

const n = 1_234;

• Optional catch binding:

try { doThing(); } catch { }.

If your target runtime supports these features, you can specify

disableESTransforms: true so that Sucrase preserves the syntax rather than

trying to transform it. Note that transpiled and standard class fields behave

slightly differently; see the

TypeScript 3.7 release notes

for details. If you use TypeScript, you can enable the TypeScript option

useDefineForClassFields to enable error checking related to these differences.

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

By default, JSX is compiled to React functions in development mode. This can be

configured with a few options:

• jsxRuntime: A string specifying the transform mode, which can be one of three values:

* "classic" (default): The original JSX transform that calls React.createElement by default.

To configure for non-React use cases, specify:

* jsxPragma: Element creation function, defaults to React.createElement.

* jsxFragmentPragma: Fragment component, defaults to React.Fragment.

* "automatic": The new JSX transform

introduced with React 17, which calls jsx functions and auto-adds import statements.

To configure for non-React use cases, specify:

* jsxImportSource: Package name for auto-generated import statements, defaults to react.

* "preserve": Don't transform JSX, and instead emit it as-is in the output code.

• production: If true, use production version of functions and don't include debugging

information. When using React in production mode with the automatic transform, this *must* be

set to true to avoid an error about jsxDEV being missing.

Legacy CommonJS interop

Two legacy modes can be used with the imports 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.

Usage

Tool integrations

• Webpack
• Gulp
• Jest
• Rollup
• Broccoli

Usage in Node

The most robust way is to use the Sucrase plugin for ts-node,

which has various Node integrations and configures Sucrase via tsconfig.json:

```bash

ts-node --transpiler sucrase/ts-node-plugin

```

For projects that don't target ESM, Sucrase also has a require hook with some

reasonable defaults that can be accessed in a few ways:

• From code: require("sucrase/register");
• When invoking Node: node -r sucrase/register main.ts
• As a separate binary: sucrase-node main.ts

Options can be passed to the require hook via a SUCRASE_OPTIONS environment

variable holding a JSON string of options.

Compiling a project to JS

For simple use cases, Sucrase comes with a sucrase CLI that mirrors your

directory structure to an output directory:

```bash

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

```

Usage from code

For any advanced use cases, Sucrase can be called from JS directly:

```js

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.

Motivation

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.

Contributing

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?