JavaScript Plugins

You can create and use custom JavaScript plugins with SWC.

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),
});