treat let handler = () => {...} the same as function handler() {...}

Author: Rich-Harris

packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/events.js

@@ -128,14 +128,16 @@ export function build_event_handler(node, metadata, context) {
 
 	// function declared in the script
 	if (handler.type === 'Identifier') {
-		const kind = context.state.scope.get(handler.name)?.declaration_kind;
-		if (kind === 'function') {
+		const binding = context.state.scope.get(handler.name);
+
+		if (binding?.is_function()) {
 			return handler;
 		}
+
 		// local variable can be assigned directly
 		// except in dev mode where when need $.apply()
 		// in order to handle warnings.
-		if (!dev && kind !== 'import') {
+		if (!dev && binding?.declaration_kind !== 'import') {
 			return handler;
 		}
 	}

packages/svelte/src/compiler/phases/scope.js

@@ -79,6 +79,21 @@ export class Binding {
 	get updated() {
 		return this.mutated || this.reassigned;
 	}
+
+	is_function() {
+		if (this.reassigned) {
+			// even if it's reassigned to another function,
+			// we can't use it directly as e.g. an event handler
+			return false;
+		}
+
+		if (this.declaration_kind === 'function') {
+			return true;
+		}
+
+		const type = this.initial?.type;
+		return type === 'ArrowFunctionExpression' || type === 'FunctionExpression';
+	}
 }
 
 export class Scope {