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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 2004 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 static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;
import static com.google.javascript.jscomp.modules.ModuleMetadataMap.ModuleMetadata;

import com.google.common.collect.ImmutableSet;
import com.google.javascript.jscomp.NodeTraversal.AbstractPostOrderCallback;
import com.google.javascript.jscomp.modules.ModuleMetadataMap;
import com.google.javascript.rhino.JSDocInfo;
import com.google.javascript.rhino.Node;
import java.util.HashSet;
import java.util.Set;

/**
 * Verifies that constants are only assigned a value once.
 * e.g. var XX = 5;
 * XX = 3;    // error!
 * XX++;      // error!
 */
// TODO(tbreisacher): Consider merging this with CheckAccessControls so that all
// const-related checks are in the same place.
class ConstCheck extends AbstractPostOrderCallback
    implements CompilerPass {

  static final DiagnosticType CONST_REASSIGNED_VALUE_ERROR =
      DiagnosticType.warning(
          "JSC_CONSTANT_REASSIGNED_VALUE_ERROR",
          "constant {0} assigned a value more than once.\n" +
          "Original definition at {1}");

  private final AbstractCompiler compiler;
  private final Set initializedConstants;
  private final ModuleMetadataMap moduleMetadataMap;
  private Set providedNames;

  /** Creates an instance. */
  public ConstCheck(AbstractCompiler compiler, ModuleMetadataMap moduleMetadataMap) {
    checkNotNull(moduleMetadataMap);
    this.compiler = compiler;
    this.initializedConstants = new HashSet<>();
    this.moduleMetadataMap = moduleMetadataMap;
  }

  @Override
  public void process(Node externs, Node root) {
    ImmutableSet.Builder providedNames = ImmutableSet.builder();
    for (ModuleMetadata metadata : this.moduleMetadataMap.getAllModuleMetadata()) {
      if (!(metadata.isGoogProvide() || metadata.isLegacyGoogModule())) {
        continue;
      }
      for (String namespace : metadata.googNamespaces()) {
        int dot = namespace.indexOf('.');
        String rootName = dot != -1 ? namespace.substring(0, dot) : namespace;
        providedNames.add(rootName);
      }
    }
    this.providedNames = providedNames.build();

    NodeTraversal.traverseRoots(compiler, this, externs, root);
  }

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    switch (n.getToken()) {
      case NAME:
        if (NodeUtil.isNameDeclaration(parent)) {
          String name = n.getString();
          Var var = t.getScope().getVar(name);
          if (isConstant(var, n)) {
            // If a constant is declared in externs, add it to initializedConstants to indicate
            // that it is initialized externally.
            if (n.isFromExterns()) {
              initializedConstants.add(var);
            } else if (n.hasChildren()) {
              if (!initializedConstants.add(var)) {
                reportError(n, var, name);
              }
            }
          }
        }
        break;

      case ASSIGN:
      case ASSIGN_BITOR:
      case ASSIGN_BITXOR:
      case ASSIGN_BITAND:
      case ASSIGN_LSH:
      case ASSIGN_RSH:
      case ASSIGN_URSH:
      case ASSIGN_ADD:
      case ASSIGN_SUB:
      case ASSIGN_MUL:
      case ASSIGN_DIV:
      case ASSIGN_MOD:
      case ASSIGN_EXPONENT:
        {
          Node lhs = n.getFirstChild();
          if (lhs.isName()) {
            String name = lhs.getString();
            Var var = t.getScope().getVar(name);
            if (isConstant(var, lhs) && !initializedConstants.add(var)) {
              reportError(n, var, name);
            } else if (var != null && var.isGoogModuleExports() && !initializedConstants.add(var)) {
              compiler.report(
                  JSError.make(n, CONST_REASSIGNED_VALUE_ERROR, "exports", n.getSourceFileName()));
            }
          }
          break;
        }

      case INC:
      case DEC:
        {
          Node lhs = n.getFirstChild();
          if (lhs.isName()) {
            String name = lhs.getString();
            Var var = t.getScope().getVar(name);
            if (isConstant(var, lhs)) {
              reportError(n, var, name);
            }
          }
          break;
        }
      default:
        break;
    }
  }

  /**
   * Gets whether a variable is a constant initialized to a literal value at the point where it is
   * declared.
   */
  private boolean isConstant(Var var, Node nameNode) {
    if (var == null) {
      checkState(
          this.providedNames.contains(nameNode.getString()),
          "Found unexpected undeclared name %s",
          nameNode);
      return false;
    }
    return var.isConst() || var.isDeclaredOrInferredConst();
  }

  /** Reports a reassigned constant error. */
  private void reportError(Node n, Var var, String name) {
    JSDocInfo info = NodeUtil.getBestJSDocInfo(n);
    if (info == null || !info.getSuppressions().contains("const")) {
      Node declNode = var.getNode();
      String declaredPosition = declNode.getSourceFileName() + ":" + declNode.getLineno();
      compiler.report(JSError.make(n, CONST_REASSIGNED_VALUE_ERROR, name, declaredPosition));
    }
  }
}