svelte/src/compiler/phases/3-transform/client/visitors/SnippetBlock.js

Cybernetically enhanced web apps

/** @import { AssignmentPattern, BlockStatement, Expression, Identifier, Statement } from 'estree' */
/** @import { AST } from '#compiler' */
/** @import { ComponentContext } from '../types' */
import { dev } from '../../../../state.js';
import { extract_paths } from '../../../../utils/ast.js';
import * as b from '../../../../utils/builders.js';
import { get_value } from './shared/declarations.js';

/**
 * @param {AST.SnippetBlock} node
 * @param {ComponentContext} context
 */
export function SnippetBlock(node, context) {
	// TODO hoist where possible
	/** @type {(Identifier | AssignmentPattern)[]} */
	const args = [b.id('$$anchor')];

	/** @type {BlockStatement} */
	let body;

	/** @type {Statement[]} */
	const declarations = [];

	if (dev) {
		declarations.push(b.stmt(b.call('$.validate_snippet_args', b.spread(b.id('arguments')))));
	}

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

	for (let i = 0; i < node.parameters.length; i++) {
		const argument = node.parameters[i];

		if (!argument) continue;

		if (argument.type === 'Identifier') {
			args.push(b.assignment_pattern(argument, b.id('$.noop')));
			transform[argument.name] = { read: b.call };

			continue;
		}

		let arg_alias = `$$arg${i}`;
		args.push(b.id(arg_alias));

		const paths = extract_paths(argument);

		for (const path of paths) {
			const name = /** @type {Identifier} */ (path.node).name;
			const needs_derived = path.has_default_value; // to ensure that default value is only called once
			const fn = b.thunk(
				/** @type {Expression} */ (context.visit(path.expression?.(b.maybe_call(b.id(arg_alias)))))
			);

			declarations.push(b.let(path.node, needs_derived ? b.call('$.derived_safe_equal', fn) : fn));

			transform[name] = {
				read: needs_derived ? get_value : b.call
			};

			// we need to eagerly evaluate the expression in order to hit any
			// 'Cannot access x before initialization' errors
			if (dev) {
				declarations.push(b.stmt(transform[name].read(b.id(name))));
			}
		}
	}

	body = b.block([
		...declarations,
		.../** @type {BlockStatement} */ (context.visit(node.body, child_state)).body
	]);

	// in dev we use a FunctionExpression (not arrow function) so we can use `arguments`
	let snippet = dev
		? b.call('$.wrap_snippet', b.id(context.state.analysis.name), b.function(null, args, body))
		: b.arrow(args, body);

	const declaration = b.const(node.expression, snippet);

	// Top-level snippets are hoisted so they can be referenced in the `<script>`
	if (context.path.length === 1 && context.path[0].type === 'Fragment') {
		if (node.metadata.can_hoist) {
			context.state.module_level_snippets.push(declaration);
		} else {
			context.state.instance_level_snippets.push(declaration);
		}
	} else {
		context.state.init.push(declaration);
	}
}