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Closure Compiler is a JavaScript optimizing compiler. It parses your JavaScript, analyzes it, removes dead code and rewrites and minimizes what's left. It also checks syntax, variable references, and types, and warns about common JavaScript pitfalls. It is used in many of Google's JavaScript apps, including Gmail, Google Web Search, Google Maps, and Google Docs.

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/*
 * Copyright 2014 The Closure Compiler Authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.javascript.jscomp;

import com.google.javascript.rhino.JSDocInfo;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;
import org.jspecify.nullness.Nullable;

/**
 * Attaches the CONST_VAR annotation to any variable that's
 * 1) Provably well-defined and assigned once in its lifetime.
 * 2) Annotated 'const'
 * 3) Declared with the 'const' keyword.
 *
 * These 3 are considered semantically equivalent. Notice that a variable
 * in a loop is never considered const.
 *
 * Note that criteria (1) is only used for normal code, not externs.
 */
class InferConsts implements CompilerPass {
  private final AbstractCompiler compiler;

  InferConsts(AbstractCompiler compiler) {
    this.compiler = compiler;
  }

  @Override
  public void process(Node externs, Node js) {
    ReferenceCollector collector =
        new ReferenceCollector(
            compiler, ReferenceCollector.DO_NOTHING_BEHAVIOR, new SyntacticScopeCreator(compiler));
    collector.process(js);

    for (Var v : collector.getAllSymbols()) {
      considerVar(v, collector.getReferences(v));
    }

    Scope globalExternsScope = new SyntacticScopeCreator(compiler).createScope(externs, null);
    for (Var v : globalExternsScope.getAllSymbols()) {
      considerVar(v, null);
    }
  }

  private void considerVar(Var v, @Nullable ReferenceCollection refCollection) {
    if (v.isImplicitGoogNamespace()) {
      return; // no name node for provided variables.
    }
    Node nameNode = v.getNameNode();
    JSDocInfo docInfo = v.getJSDocInfo();
    if (docInfo != null && docInfo.isConstant()) {
      nameNode.setDeclaredConstantVar(true);
    } else if (nameNode != null && nameNode.getParent().isConst()) {
      nameNode.setDeclaredConstantVar(true);
    }
    if (isInferredConst(v, refCollection)) {
      nameNode.setInferredConstantVar(true);
    }
  }

  private static boolean isInferredConst(Var v, ReferenceCollection refCollection) {
    Node nameNode = v.getNameNode();
    if (nameNode == null || refCollection == null || !refCollection.isAssignedOnceInLifetime()) {
      return false;
    }
    if (v.isImplicitGoogNamespace()) {
      return false;
    }
    Token declarationType = v.declarationType();

    switch (declarationType) {
      case LET:
        // Check that non-destructuring let names are actually assigned at declaration.
        return !nameNode.getParent().isLet() || nameNode.hasChildren();
      case CONST:
      case CATCH:
      case CLASS:
      case PARAM_LIST: // Parameters cannot be referenced before declaration.
      case FUNCTION: // Function hoisting means no references before declaration.
        return true;
      case IMPORT:
        // ES module exports are mutable.
        // TODO(lharker): make this check smarter if we start optimizing unrewritten modules.
        return false;
      case VAR:
        // var hoisting requires this extra work to make sure the 'declaration' is also the first
        // reference.
        return refCollection.firstReferenceIsAssigningDeclaration()
            && refCollection.isWellDefined();
      default:
        throw new IllegalStateException("Unrecognized declaration type " + declarationType);
    }
  }
}