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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 2018 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.lint;

import com.google.common.collect.ImmutableSet;
import com.google.javascript.jscomp.AbstractCompiler;
import com.google.javascript.jscomp.CompilerPass;
import com.google.javascript.jscomp.DiagnosticType;
import com.google.javascript.jscomp.NodeTraversal;
import com.google.javascript.jscomp.NodeTraversal.AbstractPostOrderCallback;
import com.google.javascript.jscomp.NodeUtil;
import com.google.javascript.rhino.JSDocInfo;
import com.google.javascript.rhino.JSTypeExpression;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.Token;

/**
 * Checks for redundant nullability modifiers.
 *
 * A nullability modifier '!' is redundant if it precedes a value type or a record literal.
 */
public class CheckRedundantNullabilityModifier extends AbstractPostOrderCallback
    implements CompilerPass {

  /** The diagnostic for a redundant nullability modifier. */
  public static final DiagnosticType REDUNDANT_NULLABILITY_MODIFIER_JSDOC =
      DiagnosticType.warning(
          "JSC_REDUNDANT_NULLABILITY_MODIFIER_JSDOC",
          "Type is already not null. Please remove the redundant '!'.");

  private static final ImmutableSet PRIMITIVE_TYPE_NAMES =
      ImmutableSet.of("boolean", "number", "string", "symbol", "undefined");

  private final AbstractCompiler compiler;

  public CheckRedundantNullabilityModifier(AbstractCompiler compiler) {
    this.compiler = compiler;
  }

  @Override
  public void process(Node externs, Node root) {
    NodeTraversal.traverseRootsEs6(compiler, this, externs, root);
  }

  @Override
  public void visit(final NodeTraversal t, final Node n, final Node p) {
    final JSDocInfo info = n.getJSDocInfo();
    if (info == null) {
      return;
    }
    if (info.hasType()) {
      checkTypeExpression(t, info.getType(), false);
    }
    for (String param : info.getParameterNames()) {
      if (info.hasParameterType(param)) {
        checkTypeExpression(t, info.getParameterType(param), false);
      }
    }
    if (info.hasReturnType()) {
      checkTypeExpression(t, info.getReturnType(), false);
    }
    if (info.hasEnumParameterType()) {
      checkTypeExpression(t, info.getEnumParameterType(), false);
    }
    if (info.hasTypedefType()) {
      checkTypeExpression(t, info.getTypedefType(), false);
    }
    // JSDocInfoParser automatically prepends a '!' to the type associated with a @this annotation.
    // It should be ignored since it doesn't actually exist in the source file.
    if (info.hasThisType()) {
      checkTypeExpression(t, info.getThisType(), true);
    }
    // We don't need to check @extends and @implements annotations since they never refer to a
    // primitive type.
    // TODO(tjgq): Should we check @throws annotations? The Google style guide forbids throwing a
    // primitive type, so the warning would be somewhat misguided.
  }

  private static void checkTypeExpression(
      final NodeTraversal t, JSTypeExpression expr, boolean isTopLevelBangAllowed) {
    NodeUtil.visitPreOrder(
        expr.getRoot(),
        node -> {
          Node parent = node.getParent();
          Node grandparent = parent != null ? parent.getParent() : null;

          boolean hasModifier = parent != null && parent.getToken() == Token.BANG;
          boolean isModifierRedundant =
              isPrimitiveType(node) || node.isFunction() || isRecordLiteral(node);
          boolean isRedundantModifierAllowed = isTopLevelBangAllowed && grandparent == null;

          if (hasModifier && isModifierRedundant && !isRedundantModifierAllowed) {
            t.report(parent, REDUNDANT_NULLABILITY_MODIFIER_JSDOC);
          }
        });
  }

  private static boolean isPrimitiveType(Node node) {
    return node.isString() && PRIMITIVE_TYPE_NAMES.contains(node.getString());
  }

  private static boolean isRecordLiteral(Node node) {
    return node.getToken() == Token.LC;
  }
}