webpack/lib/ConstPlugin.js

/*
	MIT License http://www.opensource.org/licenses/mit-license.php
	Author Tobias Koppers @sokra
*/
"use strict";
const ConstDependency = require("./dependencies/ConstDependency");
const NullFactory = require("./NullFactory");
const ParserHelpers = require("./ParserHelpers");

const getQuery = (request) => {
	const i = request.indexOf("?");
	return i !== -1 ? request.substr(i) : "";
};

const collectDeclaration = (declarations, pattern) => {
	const stack = [pattern];
	while(stack.length > 0) {
		const node = stack.pop();
		switch(node.type) {
			case "Identifier":
				declarations.add(node.name);
				break;
			case "ArrayPattern":
				for(const element of node.elements)
					if(element) stack.push(element);
				break;
			case "AssignmentPattern":
				stack.push(node.left);
				break;
			case "ObjectPattern":
				for(const property of node.properties)
					stack.push(property.value);
				break;
			case "RestElement":
				stack.push(node.argument);
				break;
		}
	}
};

const getHoistedDeclarations = (branch, includeFunctionDeclarations) => {
	const declarations = new Set();
	const stack = [branch];
	while(stack.length > 0) {
		const node = stack.pop();
		// Some node could be `null` or `undefined`.
		if(!node)
			continue;
		switch(node.type) {
			// Walk through control statements to look for hoisted declarations.
			// Some branches are skipped since they do not allow declarations.
			case "BlockStatement":
				for(const stmt of node.body)
					stack.push(stmt);
				break;
			case "IfStatement":
				stack.push(node.consequent);
				stack.push(node.alternate);
				break;
			case "ForStatement":
				stack.push(node.init);
				stack.push(node.body);
				break;
			case "ForInStatement":
			case "ForOfStatement":
				stack.push(node.left);
				stack.push(node.body);
				break;
			case "DoWhileStatement":
			case "WhileStatement":
			case "LabeledStatement":
				stack.push(node.body);
				break;
			case "SwitchStatement":
				for(const cs of node.cases)
					for(const consequent of cs.consequent)
						stack.push(consequent);
				break;
			case "TryStatement":
				stack.push(node.block);
				if(node.handler)
					stack.push(node.handler.body);
				stack.push(node.finalizer);
				break;
			case "FunctionDeclaration":
				if(includeFunctionDeclarations)
					collectDeclaration(declarations, node.id);
				break;
			case "VariableDeclaration":
				if(node.kind === "var")
					for(const decl of node.declarations)
						collectDeclaration(declarations, decl.id);
				break;
		}
	}
	return Array.from(declarations);
};

class ConstPlugin {
	apply(compiler) {
		compiler.hooks.compilation.tap("ConstPlugin", (compilation, {
			normalModuleFactory
		}) => {
			compilation.dependencyFactories.set(ConstDependency, new NullFactory());
			compilation.dependencyTemplates.set(ConstDependency, new ConstDependency.Template());

			const handler = parser => {
				parser.hooks.statementIf.tap("ConstPlugin", statement => {
					const param = parser.evaluateExpression(statement.test);
					const bool = param.asBool();
					if(typeof bool === "boolean") {
						if(statement.test.type !== "Literal") {
							const dep = new ConstDependency(`${bool}`, param.range);
							dep.loc = statement.loc;
							parser.state.current.addDependency(dep);
						}
						const branchToRemove = bool ? statement.alternate : statement.consequent;
						if(branchToRemove) {
							// Before removing the dead branch, the hoisted declarations
							// must be collected.
							//
							// Given the following code:
							//
							//     if (true) f() else g()
							//     if (false) {
							//       function f() {}
							//       const g = function g() {}
							//       if (someTest) {
							//         let a = 1
							//         var x, {y, z} = obj
							//       }
							//     } else {
							//       …
							//     }
							//
							// the generated code is:
							//
							//     if (true) f() else {}
							//     if (false) {
							//       var f, x, y, z;   (in loose mode)
							//       var x, y, z;      (in strict mode)
							//     } else {
							//       …
							//     }
							//
							// NOTE: When code runs in strict mode, `var` declarations
							// are hoisted but `function` declarations don't.
							//
							let declarations;
							if(parser.scope.isStrict) {
								// If the code runs in strict mode, variable declarations
								// using `var` must be hoisted.
								declarations = getHoistedDeclarations(branchToRemove, false);
							} else {
								// Otherwise, collect all hoisted declaration.
								declarations = getHoistedDeclarations(branchToRemove, true);
							}
							let replacement;
							if(declarations.length > 0) {
								replacement = `{ var ${declarations.join(", ")}; }`;
							} else {
								replacement = "{}";
							}
							const dep = new ConstDependency(replacement, branchToRemove.range);
							dep.loc = branchToRemove.loc;
							parser.state.current.addDependency(dep);
						}
						return bool;
					}
				});
				parser.hooks.expressionConditionalOperator.tap("ConstPlugin", expression => {
					const param = parser.evaluateExpression(expression.test);
					const bool = param.asBool();
					if(typeof bool === "boolean") {
						if(expression.test.type !== "Literal") {
							const dep = new ConstDependency(` ${bool}`, param.range);
							dep.loc = expression.loc;
							parser.state.current.addDependency(dep);
						}
						// Expressions do not hoist.
						// It is safe to remove the dead branch.
						//
						// Given the following code:
						//
						//   false ? someExpression() : otherExpression();
						//
						// the generated code is:
						//
						//   false ? undefined : otherExpression();
						//
						const branchToRemove = bool ? expression.alternate : expression.consequent;
						const dep = new ConstDependency("undefined", branchToRemove.range);
						dep.loc = branchToRemove.loc;
						parser.state.current.addDependency(dep);
						return bool;
					}
				});
				parser.hooks.evaluateIdentifier.for("__resourceQuery").tap("ConstPlugin", expr => {
					if(!parser.state.module) return;
					return ParserHelpers.evaluateToString(getQuery(parser.state.module.resource))(expr);
				});
				parser.hooks.expression.for("__resourceQuery").tap("ConstPlugin", () => {
					if(!parser.state.module) return;
					parser.state.current.addVariable("__resourceQuery", JSON.stringify(getQuery(parser.state.module.resource)));
					return true;
				});
			};

			normalModuleFactory.hooks.parser.for("javascript/auto").tap("ConstPlugin", handler);
			normalModuleFactory.hooks.parser.for("javascript/dynamic").tap("ConstPlugin", handler);
			normalModuleFactory.hooks.parser.for("javascript/esm").tap("ConstPlugin", handler);
		});
	}
}

module.exports = ConstPlugin;