Compilation

Compilation works out of the box with SWC and does not require customization. Optionally, you can override the configuration. Here are the defaults:

{
  "jsc": {
    "parser": {
      "syntax": "ecmascript",
      "jsx": false,
      "dynamicImport": false,
      "privateMethod": false,
      "functionBind": false,
      "exportDefaultFrom": false,
      "exportNamespaceFrom": false,
      "decorators": false,
      "decoratorsBeforeExport": false,
      "topLevelAwait": false,
      "importMeta": false,
      "preserveAllComments": false
    },
    "transform": null,
    "target": "es5",
    "loose": false,
    "externalHelpers": false,
    // Requires v1.2.50 or upper and requires target to be es2016 or upper.
    "keepClassNames": false
  },
  "isModule": false
}

jsc.externalHelpers

{
  "jsc": {
    "externalHelpers": true
  }
}

The output code may depend on helper functions to support the target environment. By default, a helper function is inlined into the output files where it is required.

You can use helpers from an external module by enabling externalHelpers and the helpers code will be imported by the output files from node_modules/@swc/helpers.

While bundling, this option will greatly reduce your file size.

You must add @swc/helpers as a dependency in addition to @swc/core.

jsc.parser

typescript

{
  "jsc": {
    "parser": {
      "syntax": "typescript",
      "tsx": false,
      "decorators": false,
      "dynamicImport": false
    }
  }
}

ecmascript

{
  "jsc": {
    "parser": {
      "syntax": "ecmascript",
      "jsx": false,
      "dynamicImport": false,
      "privateMethod": false,
      "functionBind": false,
      "classPrivateProperty": false,
      "exportDefaultFrom": false,
      "exportNamespaceFrom": false,
      "decorators": false,
      "decoratorsBeforeExport": false,
      "importMeta": false
    }
  }
}

jsc.target

Starting from @swc/core v1.0.27, you can specify the target environment by using the field.

{
  "jsc": {
    // Disable es3 / es5 / es2015 transforms
    "target": "es2016"
  }
}

jsc.loose

Starting from @swc/core v1.1.4, you can enable "loose" transformations by enabling jsc.loose which works like babel-preset-env loose mode.

{
  "jsc": {
    "loose": true
  }
}

jsc.transform

{
  "jsc": {
    "transform": {
      "react": {
        "pragma": "React.createElement",
        "pragmaFrag": "React.Fragment",
        "throwIfNamespace": true,
        "development": false,
        "useBuiltins": false
      },
      "optimizer": {
        "globals": {
          "vars": {
            "__DEBUG__": "true"
          }
        }
      }
    }
  }
}

jsc.transform.legacyDecorator

You can use the legacy (stage 1) class decorators syntax and behavior.

{
  "jsc": {
    "parser": {
      "syntax": "ecmascript",
      "decorators": true
    },
    "transform": {
      "legacyDecorator": true
    }
  }
}

jsc.transform.decoratorMetadata

This feature requires v1.2.13+.

If you are using typescript and decorators with emitDecoratorMetadata enabled, you can use swc for faster iteration:

{
  "jsc": {
    "parser": {
      "syntax": "typescript",
      "decorators": true
    },
    "transform": {
      "legacyDecorator": true,
      "decoratorMetadata": true
    }
  }
}

jsc.transform.react

jsc.transform.react.runtime

Possible values: automatic, classic. This affects how JSX source code will be compiled.

Use runtime: automatic to use a JSX runtime module (e.g. react/jsx-runtime introduced in React 17).
Use runtime: classic to use React.createElement instead - with this option, you must ensure that React is in scope when using JSX.

Learn more here.

jsc.transform.react.importSource

Defaults to react.
When using runtime: automatic, determines the runtime library to import.
This option can be overrided with @jsxImportSource foo.

jsc.transform.react.pragma

Defaults to React.createElement.
When using runtime: classic, replaces the function used when compiling JSX expressions.
This option can be overrided with @jsx foo.

jsc.transform.react.pragmaFrag

Defaults to React.Fragment
Replace the component used when compiling JSX fragments.
This option can be overrided with @jsxFrag foo.

jsc.transform.react.throwIfNamespace

Toggles whether or not to throw an error if an XML namespaced tag name is used. For example: <f:image />

Though the JSX spec allows this, it is disabled by default since React's JSX does not currently have support for it.

jsc.transform.react.development

Toggles debug props __self and __source on elements generated from JSX, which are used by development tooling such as React Developer Tools.

This option is set automatically based on the Webpack mode setting when used with swc-loader. See Using swc with webpack.

jsc.transform.react.useBuiltins

Use Object.assign() instead of _extends. Defaults to false.

jsc.transform.react.refresh

Enable react-refresh related transform. Defaults to false as it's considered experimental.

Pass refresh: true to enable this feature, or an object with the following:

interface ReactRefreshConfig {
  refreshReg: String;
  refreshSig: String;
  emitFullSignatures: boolean;
}

jsc.transform.constModules

{
  "jsc": {
    "transform": {
      "constModules": {
        "globals": {
          "@ember/env-flags": {
            "DEBUG": "true"
          },
          "@ember/features": {
            "FEATURE_A": "false",
            "FEATURE_B": "true"
          }
        }
      }
    }
  }
}

Then, source code like:

import { DEBUG } from "@ember/env-flags";
import { FEATURE_A, FEATURE_B } from "@ember/features";

console.log(DEBUG, FEATURE_A, FEATURE_B);

is transformed to:

console.log(true, false, true);

jsc.transform.optimizer

The SWC optimizier assumes:

It's a module or wrapped in an iife.
Accessing (get) global variables does not have a side-effect. It is the same assumption as the google closure compiler.
You don't add fields to literals like a numeric literal, regular expression or a string literal.
Files are served as gzipped.

SWC will not focus on reducing the size of non-gzipped file size.

Setting this to undefined skips optimizer pass.

jsc.transform.optimizer.simplify

Requires v1.2.101+

You can set this to false to use inline_globals while skipping optimizations.

{
  "jsc": {
    "transform": {
      "optimizer": {
        "simplify": false,
        "globals": {
          "vars": {
            "__DEBUG__": "true"
          }
        }
      }
    }
  }
}

jsc.transform.optimizer.globals

Requires v1.2.101+

vars - Variables to inline.
typeofs - If you set { "window": "object" }, typeof window will be replaced with "object".

{
  "jsc": {
    "transform": {
      "optimizer": {
        "globals": {
          "vars": {
            "__DEBUG__": "true"
          }
        }
      }
    }
  }
}

Then, you can use it like npx swc '__DEBUG__' --filename input.js.

jsc.transform.optimizer.jsonify

Requires v1.1.1+

minCost - If cost of parsing a pure object literal is larger than this value, the object literal is converted to JSON.parse('{"foo": "bar"}'). Defaults to 1024.

{
  "jsc": {
    "transform": {
      "optimizer": {
        "jsonify": {
          "minCost": 0
        }
      }
    }
  }
}

This will change all pure object literals to JSON.parse("").

jsc.keepClassNames

Requires v1.2.50+ and target to be es2016 or higher

Enabling this option will make swc preserve original class names.

jsc.paths

Requires v1.2.62+

Syntax is identical as it of tsconfig.json: learn more.

Requires jsc.baseUrl. See below.

jsc.minify

Requires v1.2.67+

See the documentation for minification for more details.

jsc.experimental

jsc.experimental.keepImportAssertions

Preserve import assertions. This is experimental because import assertions are not covered by ecmascript specifications yet.

jsc.experimental.plugins

It follows resolving rule of node.js,.

Specify the plugin name like

{
  "jsc": {
    "experimental": {
      "plugins": [
        ["@swc/plugin-styled-jsx", {}]
      ]
    }
  }
}

styled-jsx works because it's published as @swc/plugin-styled-jsx.

Indicate that all comments should be preserved during compilation. Comments from source may be shifted in order to preserve thier relative location from source to compiled output. This feature is useful for transpilation that requires comments remain relatively close to the source: e.g. files under test with istanbul-ignore coverage annotations.

Multiple Entries

Requires v1.0.47+

[
  {
    "test": ".*\\.js$",
    "module": {
      "type": "commonjs"
    }
  },
  {
    "test": ".*\\.ts$",
    "module": {
      "type": "amd"
    }
  }
]

This make SWC compile JavaScript files as CommonJS modules and compile TypeScript files as AMD modules.

Note that test option can be used to transcompile only typescript files, like

{
  "test": ".*\\.ts$",
  "jsc": {
    "parser": {
      "syntax": "typescript",
      "tsx": false,
      "decorators": true,
      "dynamicImport": true
    }
  }
}

test

Type: Regex / Regex[]

{
  "test": ".*\\.ts$",
  "jsc": {
    "parser": {
      "syntax": "typescript",
      "tsx": false,
      "decorators": true,
      "dynamicImport": true
    }
  }
}

exclude

Type: Regex / Regex[]

{
  "exclude": [".*\\.js$", ".*\\.map$"],
  "jsc": {
    "parser": {
      "syntax": "typescript",
      "tsx": false,
      "decorators": true,
      "dynamicImport": true
    }
  }
}

sourceMaps

Requires v1.2.50+

Enable source map by adding sourceMaps: true or sourceMaps: 'inline' to the .swcrc.

{
  "sourceMaps": true
}

inlineSourcesContent

Requires v1.2.101+

Defaults to true. If you want to make swc store contents of files into sourcemap, you can set inlineSourcesContent to true.

{
  "sourceMaps": true,
  "inlineSourcesContent": true
}

isModule

Possible values: true, false, "unknown"

Used to treat input as a module or script. If this is set to unknown, it will be Module if it's esm and Script otherwise.

Last updated on September 22, 2022