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Plugins (v1)

You can create and use custom plugins with SWC.

Warning: Passing the AST to the JS Plugin system is a performance bottleneck.

Therefore, we are considering two approaches for the next SWC major version:

  1. Creating a plugin system in Rust (#2337)
  2. Solve the bottleneck to keep a JS plugin system (#2175)

Example

This example, plugin-strip-console, replaces all console.log statements with void 0.

You can turn on optimizer with jsc.transform.optimizer to remove void 0.

import { CallExpression, Expression, transformSync } from "@swc/core";
import { Visitor } from "@swc/core/Visitor.js";

class ConsoleStripper extends Visitor {
  visitCallExpression(expression: CallExpression): Expression {
    if (expression.callee.type !== "MemberExpression") {
      return expression;
    }

    if (
      expression.callee.object.type === "Identifier" &&
      expression.callee.object.value === "console"
    ) {
      if (expression.callee.property.type === "Identifier") {
        return {
          type: "UnaryExpression",
          span: expression.span,
          operator: "void",
          argument: {
            type: "NumericLiteral",
            span: expression.span,
            value: 0,
          },
        };
      }
    }

    return expression;
  }
}

const out = transformSync(
  `
if (foo) {
    console.log("Foo")
} else {
    console.log("Bar")
}`,
  {
    plugin: (m) => new ConsoleStripper().visitProgram(m),
  }
);

out.code ===
  `if (foo) {
    void 0;
} else {
    void 0;
}`;

Visitor API

Visitors are a pattern used in AST traversal across languages. They are an object with methods defined for accepting particular node types in a tree. That's a bit abstract, so let's look at an example:

/// @ts-check
import { Visitor } from "@swc/core/Visitor.js";

class MyVisitor extends Visitor {
  visitIdentifier(node) {
    console.log("called")
    return node;
  }
}

This visitor will call the visitIdentifier() method for every Identifier in the tree when used during a traversal.

With the code below, the visitIdentifier() method will be called four times with each Identifier (including square):

import { transformSync } from "@swc/core";

transformSync(`
  function square(n) {
    return n * n;
  }
`, {
  plugin: MyVisitor,
});

// Called!
// Called!
// Called!
// Called!

By extending @swc/core/Visitor, you can choose from a variety of visitors:

  • visitProgram(n: Program): Program;
  • visitModule(m: Module): Module;
  • visitScript(m: Script): Script;
  • visitModuleItems(items: ModuleItem[]): ModuleItem[];
  • visitModuleItem(n: ModuleItem): ModuleItem;
  • visitModuleDeclaration(n: ModuleDeclaration): ModuleDeclaration;
  • visitTsNamespaceExportDeclaration(n: TsNamespaceExportDeclaration): ModuleDeclaration;
  • visitTsExportAssignment(n: TsExportAssignment): TsExportAssignment;
  • visitTsImportEqualsDeclaration(n: TsImportEqualsDeclaration): ModuleDeclaration;
  • visitTsModuleReference(n: TsModuleReference): TsModuleReference;
  • visitTsExternalModuleReference(n: TsExternalModuleReference): TsExternalModuleReference;
  • visitExportAllDeclration(n: ExportAllDeclaration): ModuleDeclaration;
  • visitExportDefaultExpression(n: ExportDefaultExpression): ModuleDeclaration;
  • visitExportNamedDeclration(n: ExportNamedDeclaration): ModuleDeclaration;
  • visitExportSpecifiers(nodes: ExportSpecifier[]): ExportSpecifier[];
  • visitExportSpecifier(n: ExportSpecifier): ExportSpecifier;
  • visitNamedExportSpecifier(n: NamedExportSpecifier): ExportSpecifier;
  • visitExportNamespaceSpecifier(n: ExportNamespaceSpecifier): ExportSpecifier;
  • visitExportDefaultSpecifier(n: ExportDefaultSpecifier): ExportSpecifier;
  • visitOptionalStringLiteral(n: StringLiteral | undefined): StringLiteral | undefined;
  • visitExportDefaultDeclaration(n: ExportDefaultDeclaration): ModuleDeclaration;
  • visitDefaultDeclaration(n: DefaultDecl): DefaultDecl;
  • visitFunctionExpression(n: FunctionExpression): FunctionExpression;
  • visitClassExpression(n: ClassExpression): ClassExpression;
  • visitExportDeclaration(n: ExportDeclaration): ModuleDeclaration;
  • visitArrayExpression(e: ArrayExpression): Expression;
  • visitArrayElement(e: ExprOrSpread | undefined): ExprOrSpread | undefined;
  • visitExprOrSpread(e: ExprOrSpread): ExprOrSpread;
  • visitSpreadElement(e: SpreadElement): SpreadElement;
  • visitOptionalExpression(e: Expression | undefined): Expression | undefined;
  • visitArrowFunctionExpression(e: ArrowFunctionExpression): Expression;
  • visitArrowBody(body: BlockStatement | Expression): BlockStatement | Expression;
  • visitBlockStatement(block: BlockStatement): BlockStatement;
  • visitStatements(stmts: Statement[]): Statement[];
  • visitStatement(stmt: Statement): Statement;
  • visitSwitchStatement(stmt: SwitchStatement): Statement;
  • visitSwitchCases(cases: SwitchCase[]): SwitchCase[];
  • visitSwitchCase(c: SwitchCase): SwitchCase;
  • visitIfStatement(stmt: IfStatement): Statement;
  • visitOptionalStatement(stmt: Statement | undefined): Statement | undefined;
  • visitBreakStatement(stmt: BreakStatement): Statement;
  • visitWhileStatement(stmt: WhileStatement): Statement;
  • visitTryStatement(stmt: TryStatement): Statement;
  • visitCatchClause(handler: CatchClause | undefined): CatchClause | undefined;
  • visitThrowStatement(stmt: ThrowStatement): Statement;
  • visitReturnStatement(stmt: ReturnStatement): Statement;
  • visitLabeledStatement(stmt: LabeledStatement): Statement;
  • visitForStatement(stmt: ForStatement): Statement;
  • visitForOfStatement(stmt: ForOfStatement): Statement;
  • visitForInStatement(stmt: ForInStatement): Statement;
  • visitEmptyStatement(stmt: EmptyStatement): Statement;
  • visitDoWhileStatement(stmt: DoWhileStatement): Statement;
  • visitDebuggerStatement(stmt: DebuggerStatement): Statement;
  • visitWithStatement(stmt: WithStatement): Statement;
  • visitDeclaration(decl: Declaration): Declaration;
  • visitVariableDeclaration(n: VariableDeclaration): VariableDeclaration;
  • visitVariableDeclarators(nodes: VariableDeclarator[]): VariableDeclarator[];
  • visitVariableDeclarator(n: VariableDeclarator): VariableDeclarator;
  • visitTsTypeAliasDeclaration(n: TsTypeAliasDeclaration): Declaration;
  • visitTsModuleDeclaration(n: TsModuleDeclaration): Declaration;
  • visitTsModuleName(n: TsModuleName): TsModuleName;
  • visitTsNamespaceBody(n: TsNamespaceBody): TsNamespaceBody | undefined;
  • visitTsNamespaceDeclaration(n: TsNamespaceDeclaration): TsModuleBlock | TsNamespaceDeclaration;
  • visitTsModuleBlock(n: TsModuleBlock): TsModuleBlock | TsNamespaceDeclaration;
  • visitTsInterfaceDeclaration(n: TsInterfaceDeclaration): TsInterfaceDeclaration;
  • visitTsInterfaceBody(n: TsInterfaceBody): TsInterfaceBody;
  • visitTsTypeElements(nodes: TsTypeElement[]): TsTypeElement[];
  • visitTsTypeElement(n: TsTypeElement): TsTypeElement;
  • visitTsEnumDeclaration(n: TsEnumDeclaration): Declaration;
  • visitTsEnumMembers(nodes: TsEnumMember[]): TsEnumMember[];
  • visitTsEnumMember(n: TsEnumMember): TsEnumMember;
  • visitTsEnumMemberId(n: TsEnumMemberId): TsEnumMemberId;
  • visitFunctionDeclaration(decl: FunctionDeclaration): Declaration;
  • visitClassDeclartion(decl: ClassDeclaration): Declaration;
  • visitClassBody(members: ClassMember[]): ClassMember[];
  • visitClassMember(member: ClassMember): ClassMember;
  • visitTsIndexSignature(n: TsIndexSignature): ClassMember;
  • visitTsFnParameters(params: TsFnParameter[]): TsFnParameter[];
  • visitTsFnParameter(n: TsFnParameter): TsFnParameter;
  • visitPrivateProperty(n: PrivateProperty): ClassMember;
  • visitPrivateMethod(n: PrivateMethod): ClassMember;
  • visitPrivateName(n: PrivateName): PrivateName;
  • visitConstructor(n: Constructor): ClassMember;
  • visitConstructorParameters(nodes: (Param | TsParameterProperty)[]): (Param | TsParameterProperty)[];
  • visitConstructorParameter(n: Param | TsParameterProperty): Param | TsParameterProperty;
  • visitTsParameterProperty(n: TsParameterProperty): TsParameterProperty | Param;
  • visitTsParameterPropertyParameter(n: TsParameterPropertyParameter): TsParameterPropertyParameter;
  • visitPropertyName(key: PropertyName): PropertyName;
  • visitAccessibility(n: Accessibility | undefined): Accessibility | undefined;
  • visitClassProperty(n: ClassProperty): ClassMember;
  • visitClassMethod(n: ClassMethod): ClassMember;
  • visitPropertName(n: PropertyName): PropertyName;
  • visitComputedPropertyKey(n: ComputedPropName): ComputedPropName;
  • visitClass(n: T): T;
  • visitFunction(n: T): T;
  • visitTsExpressionsWithTypeArguments(nodes: TsExpressionWithTypeArguments[]): TsExpressionWithTypeArguments[];
  • visitTsExpressionWithTypeArguments(n: TsExpressionWithTypeArguments): TsExpressionWithTypeArguments;
  • visitTsTypeParameterInstantiation(n: TsTypeParameterInstantiation | undefined): TsTypeParameterInstantiation | undefined;
  • visitTsTypes(nodes: TsType[]): TsType[];
  • visitTsEntityName(n: TsEntityName): TsEntityName;
  • visitTsQualifiedName(n: TsQualifiedName): TsQualifiedName;
  • visitDecorators(nodes: Decorator[] | undefined): Decorator[] | undefined;
  • visitDecorator(n: Decorator): Decorator;
  • visitExpressionStatement(stmt: ExpressionStatement): Statement;
  • visitContinueStatement(stmt: ContinueStatement): Statement;
  • visitExpression(n: Expression): Expression;
  • visitOptionalChainingExpression(n: OptionalChainingExpression): Expression;
  • visitAssignmentExpression(n: AssignmentExpression): Expression;
  • visitPatternOrExpressison(n: Pattern | Expression): Pattern | Expression;
  • visitYieldExpression(n: YieldExpression): Expression;
  • visitUpdateExpression(n: UpdateExpression): Expression;
  • visitUnaryExpression(n: UnaryExpression): Expression;
  • visitTsTypeAssertion(n: TsTypeAssertion): Expression;
  • visitTsConstAssertion(n: TsConstAssertion): Expression;
  • visitTsNonNullExpression(n: TsNonNullExpression): Expression;
  • visitTsAsExpression(n: TsAsExpression): Expression;
  • visitThisExpression(n: ThisExpression): Expression;
  • visitTemplateLiteral(n: TemplateLiteral): Expression;
  • visitParameters(n: Param[]): Param[];
  • visitParameter(n: Param): Param;
  • visitTaggedTemplateExpression(n: TaggedTemplateExpression): Expression;
  • visitSequenceExpression(n: SequenceExpression): Expression;
  • visitRegExpLiteral(n: RegExpLiteral): Expression;
  • visitParenthesisExpression(n: ParenthesisExpression): Expression;
  • visitObjectExpression(n: ObjectExpression): Expression;
  • visitObjectProperties(nodes: (Property | SpreadElement)[]): (Property | SpreadElement)[];
  • visitObjectProperty(n: Property | SpreadElement): Property | SpreadElement;
  • visitProperty(n: Property): Property | SpreadElement;
  • visitSetterProperty(n: SetterProperty): Property | SpreadElement;
  • visitMethodProperty(n: MethodProperty): Property | SpreadElement;
  • visitKeyValueProperty(n: KeyValueProperty): Property | SpreadElement;
  • visitGetterProperty(n: GetterProperty): Property | SpreadElement;
  • visitAssignmentProperty(n: AssignmentProperty): Property | SpreadElement;
  • visitNullLiteral(n: NullLiteral): NullLiteral;
  • visitNewExpression(n: NewExpression): Expression;
  • visitTsTypeArguments(n: TsTypeParameterInstantiation | undefined): TsTypeParameterInstantiation | undefined;
  • visitArguments(nodes: Argument[]): Argument[];
  • visitArgument(n: Argument): Argument;
  • visitMetaProperty(n: MetaProperty): Expression;
  • visitMemberExpression(n: MemberExpression): Expression;
  • visitExpressionOrSuper(n: Expression | Super): Expression | Super;
  • visitJSXText(n: JSXText): JSXText;
  • visitJSXNamespacedName(n: JSXNamespacedName): JSXNamespacedName;
  • visitJSXMemberExpression(n: JSXMemberExpression): JSXMemberExpression;
  • visitJSXObject(n: JSXObject): JSXObject;
  • visitJSXFragment(n: JSXFragment): JSXFragment;
  • visitJSXClosingFragment(n: JSXClosingFragment): JSXClosingFragment;
  • visitJSXElementChildren(nodes: JSXElementChild[]): JSXElementChild[];
  • visitJSXElementChild(n: JSXElementChild): JSXElementChild;
  • visitJSXExpressionContainer(n: JSXExpressionContainer): JSXExpressionContainer;
  • visitJSXSpreadChild(n: JSXSpreadChild): JSXElementChild;
  • visitJSXOpeningFragment(n: JSXOpeningFragment): JSXOpeningFragment;
  • visitJSXEmptyExpression(n: JSXEmptyExpression): Expression;
  • visitJSXElement(n: JSXElement): JSXElement;
  • visitJSXClosingElement(n: JSXClosingElement | undefined): JSXClosingElement | undefined;
  • visitJSXElementName(n: JSXElementName): JSXElementName;
  • visitJSXOpeningElement(n: JSXOpeningElement): JSXOpeningElement;
  • visitJSXAttributes(attrs: JSXAttributeOrSpread[] | undefined): JSXAttributeOrSpread[] | undefined;
  • visitJSXAttributeOrSpread(n: JSXAttributeOrSpread): JSXAttributeOrSpread;
  • visitJSXAttribute(n: JSXAttribute): JSXAttributeOrSpread;
  • visitJSXAttributeValue(n: JSXAttrValue | undefined): JSXAttrValue | undefined;
  • visitJSXAttributeName(n: JSXAttributeName): JSXAttributeName;
  • visitConditionalExpression(n: ConditionalExpression): Expression;
  • visitCallExpression(n: CallExpression): Expression;
  • visitBooleanLiteral(n: BooleanLiteral): BooleanLiteral;
  • visitBinaryExpression(n: BinaryExpression): Expression;
  • visitAwaitExpression(n: AwaitExpression): Expression;
  • visitTsTypeParameterDeclaration(n: TsTypeParameterDeclaration | undefined): TsTypeParameterDeclaration | undefined;
  • visitTsTypeParameters(nodes: TsTypeParameter[]): TsTypeParameter[];
  • visitTsTypeParameter(n: TsTypeParameter): TsTypeParameter;
  • visitTsTypeAnnotation(a: TsTypeAnnotation | undefined): TsTypeAnnotation | undefined;
  • visitTsType(n: TsType): TsType;
  • visitPatterns(nodes: Pattern[]): Pattern[];
  • visitImportDeclaration(n: ImportDeclaration): ImportDeclaration;
  • visitImportSpecifiers(nodes: ImportSpecifier[]): ImportSpecifier[];
  • visitImportSpecifier(node: ImportSpecifier): ImportSpecifier;
  • visitNamedImportSpecifier(node: NamedImportSpecifier): NamedImportSpecifier;
  • visitImportNamespaceSpecifier(node: ImportNamespaceSpecifier): ImportNamespaceSpecifier;
  • visitImportDefaultSpecifier(node: ImportDefaultSpecifier): ImportSpecifier;
  • visitBindingIdentifier(i: Identifier): Identifier;
  • visitIdentifierReference(i: Identifier): Identifier;
  • visitLabelIdentifier(label: Identifier): Identifier;
  • visitIdentifier(n: Identifier): Identifier;
  • visitStringLiteral(n: StringLiteral): StringLiteral;
  • visitNumericLiteral(n: NumericLiteral): NumericLiteral;
  • visitPattern(n: Pattern): Pattern;
  • visitRestElement(n: RestElement): RestElement;
  • visitAssignmentPattern(n: AssignmentPattern): Pattern;
  • visitObjectPattern(n: ObjectPattern): Pattern;
  • visitObjectPatternProperties(nodes: ObjectPatternProperty[]): ObjectPatternProperty[];
  • visitObjectPatternProperty(n: ObjectPatternProperty): ObjectPatternProperty;
  • visitKeyValuePatternProperty(n: KeyValuePatternProperty): ObjectPatternProperty;
  • visitAssignmentPatternProperty(n: AssignmentPatternProperty): ObjectPatternProperty;
  • visitArrayPattern(n: ArrayPattern): Pattern;
  • visitArrayPatternElements(nodes: (Pattern | undefined)[]): (Pattern | undefined)[];
  • visitArrayPatternElement(n: Pattern | undefined): Pattern | undefined;

Using Multiple Plugins

You can use plugins to compose multiple plugins together:

import { transformSync, plugins } from "@swc/core";

const out = transformSync(src, {
  plugin: plugins(pluginA, pluginB),
});
Last updated on June 22, 2022
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