ConstTag() — svelte Function Reference

Source Code

packages/svelte/src/compiler/phases/3-transform/client/visitors/ConstTag.js lines 16–87

export function ConstTag(node, context) {
	const declaration = node.declaration.declarations[0];
	// TODO we can almost certainly share some code with $derived(...)
	if (declaration.id.type === 'Identifier') {
		const init = build_expression(context, declaration.init, node.metadata.expression);

		let expression = create_derived(context.state, init, node.metadata.expression.has_await);

		if (dev) {
			expression = b.call('$.tag', expression, b.literal(declaration.id.name));
		}

		context.state.transform[declaration.id.name] = { read: get_value };

		add_const_declaration(
			context.state,
			declaration.id,
			expression,
			node.metadata.expression,
			context.state.scope.get_bindings(declaration)
		);
	} else {
		const identifiers = extract_identifiers(declaration.id);
		const tmp = b.id(context.state.scope.generate('computed_const'));

		const transform = { ...context.state.transform };

		// Make all identifiers that are declared within the following computed regular
		// variables, as they are not signals in that context yet
		for (const node of identifiers) {
			delete transform[node.name];
		}

		const child_state = /** @type {ComponentContext['state']} */ ({
			...context.state,
			transform
		});

		// TODO optimise the simple `{ x } = y` case — we can just return `y`
		// instead of destructuring it only to return a new object
		const init = build_expression(
			{ ...context, state: child_state },
			declaration.init,
			node.metadata.expression
		);

		const block = b.block([
			b.const(/** @type {Pattern} */ (context.visit(declaration.id, child_state)), init),
			b.return(b.object(identifiers.map((node) => b.prop('init', node, node))))
		]);

		let expression = create_derived(context.state, block, node.metadata.expression.has_await);

		if (dev) {
			expression = b.call('$.tag', expression, b.literal('[@const]'));
		}

		add_const_declaration(
			context.state,
			tmp,
			expression,
			node.metadata.expression,
			context.state.scope.get_bindings(declaration)
		);

		for (const node of identifiers) {
			context.state.transform[node.name] = {
				read: (node) => b.member(b.call('$.get', tmp), node)
			};
		}
	}
}