Sucrase
Try it out
Quick usage
yarn add --dev sucrase # Or npm install --save-dev sucrase
node -r sucrase/register main.ts
Using the ts-node integration:
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:
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
,
_jsx()
by setting the jsxRuntime
option.
Also adds createReactClass
display names and JSX context information.- typescript: Compiles TypeScript code to JavaScript, removing type
isolatedModules
TypeScript flag so that the typechecker will disallow the few features like
const enum
s 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
- jest: Hoist desired jest method calls above imports in
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; }
.
#x
private field syntax.
const n = 1_234;
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,
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
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 transformintroduced 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
jsxDEV
being missing.Legacy CommonJS interop
Two legacy modes can be used with theimports
transform:- enableLegacyTypeScriptModuleInterop: Use the default TypeScript approach
--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
require('./MyModule')
instead of
require('./MyModule').default
. Analogous to
babel-plugin-add-module-exports.Usage
Tool integrations
Usage in Node
The most robust way is to use the Sucrase plugin for ts-node, which has various Node integrations and configures Sucrase viatsconfig.json
:
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 asucrase
CLI that mirrors your
directory structure to an output directory:
sucrase ./srcDir -d ./outDir --transforms typescript,imports
Usage from code
For any advanced use cases, Sucrase can be called 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
- Sucrase is not pluginizable. With the current architecture, transforms need to
- Sucrase is not good for prototyping language extensions and upcoming language
- Sucrase will never produce code for old browsers like IE. Compiling code down
- Sucrase is hesitant to implement upcoming JS features, although some of them
- Like Babel, Sucrase is not a typechecker, and must process each file in
const enum
s are treated as regular
enum
s rather than inlining across files.- You should think carefully before using Sucrase in production. Sucrase is
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:- Tokenize the input source code into a token stream.
- Parse the token stream into an AST.
- Walk the AST to compute the scope information for each variable.
- Apply all transform plugins in a single traversal, resulting in a new AST.
- 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:
- 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.
- Scan through the tokens, computing preliminary information like all
imported/exported names.
- 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.