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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. This binary checks for style issues such as incorrect or missing JSDoc usage, and missing goog.require() statements. It does not do more advanced checks such as typechecking.

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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.lint;

import com.google.common.base.Predicate;
import com.google.common.base.Predicates;
import com.google.javascript.jscomp.AbstractCompiler;
import com.google.javascript.jscomp.CheckPathsBetweenNodes;
import com.google.javascript.jscomp.ControlFlowGraph;
import com.google.javascript.jscomp.DiagnosticType;
import com.google.javascript.jscomp.HotSwapCompilerPass;
import com.google.javascript.jscomp.NodeTraversal;
import com.google.javascript.jscomp.NodeUtil;
import com.google.javascript.jscomp.graph.DiGraph.DiGraphEdge;
import com.google.javascript.rhino.FunctionTypeI;
import com.google.javascript.rhino.JSDocInfo;
import com.google.javascript.rhino.Node;
import com.google.javascript.rhino.TypeI;

/**
 * Checks when a function is annotated as returning {SomeType} (nullable)
 * but actually always returns {!SomeType}, i.e. never returns null.
 *
 */
public final class CheckNullableReturn implements HotSwapCompilerPass, NodeTraversal.Callback {
  final AbstractCompiler compiler;

  public static final DiagnosticType NULLABLE_RETURN =
      DiagnosticType.disabled(
          "JSC_NULLABLE_RETURN",
          "This function''s return type is nullable, but it always returns a "
          + "non-null value. Consider making the return type non-nullable.");

  public static final DiagnosticType NULLABLE_RETURN_WITH_NAME =
      DiagnosticType.disabled(
          "JSC_NULLABLE_RETURN_WITH_NAME",
          "The return type of the function \"{0}\" is nullable, but it always "
          + "returns a non-null value. Consider making the return type "
          + "non-nullable.");

  private static final Predicate NULLABLE_RETURN_PREDICATE =
      new Predicate() {
    @Override
    public boolean apply(Node input) {
      // Check for null because the control flow graph's implicit return node is
      // represented by null, so this value might be input.
      if (input == null || !input.isReturn()) {
        return false;
      }
      Node returnValue = input.getFirstChild();
      return returnValue != null && isNullable(returnValue);
    }
  };

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

  public static boolean hasReturnDeclaredNullable(Node n) {
    return n.isNormalBlock()
        && n.hasChildren()
        && isReturnTypeNullable(n.getParent())
        && !hasSingleThrow(n);
  }

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    // Do the checks when 'n' is the block node and 'parent' is the function
    // node, so that getControlFlowGraph will return the graph inside
    // the function, rather than the graph of the enclosing scope.
    if (hasReturnDeclaredNullable(n)
        && !canReturnNull(t.getControlFlowGraph())) {
      String fnName = NodeUtil.getNearestFunctionName(parent);
      if (fnName != null && !fnName.isEmpty()) {
        compiler.report(t.makeError(parent, NULLABLE_RETURN_WITH_NAME, fnName));
      } else {
        compiler.report(t.makeError(parent, NULLABLE_RETURN));
      }
    }
  }

  /**
   * @return whether the blockNode contains only a single "throw" child node.
   */
  private static boolean hasSingleThrow(Node blockNode) {
    if (blockNode.hasOneChild() && blockNode.getFirstChild().isThrow()) {
      // Functions consisting of a single "throw FOO" can be actually abstract,
      // so do not check their return type nullability.
      return true;
    }

    return false;
  }

  /**
   * @return True if n is a function node which is explicitly annotated
   * as returning a nullable type, other than {?}.
   */
  private static boolean isReturnTypeNullable(Node n) {
    if (n == null || !n.isFunction()) {
      return false;
    }
    FunctionTypeI functionType = n.getTypeI().toMaybeFunctionType();
    if (functionType == null) {
      // If the JSDoc declares a non-function type on a function node, we still shouldn't crash.
      return false;
    }
    TypeI returnType = functionType.getReturnType();
    if (returnType == null
        || returnType.isUnknownType() || !returnType.isNullable()) {
      return false;
    }
    JSDocInfo info = NodeUtil.getBestJSDocInfo(n);
    return info != null && info.hasReturnType();
  }

  /**
   * @return True if the given ControlFlowGraph could return null.
   */
  public static boolean canReturnNull(ControlFlowGraph graph) {
    CheckPathsBetweenNodes test = new CheckPathsBetweenNodes<>(graph,
        graph.getEntry(), graph.getImplicitReturn(), NULLABLE_RETURN_PREDICATE,
        Predicates.>alwaysTrue());

    return test.somePathsSatisfyPredicate();
  }

  /**
   * @return True if the node represents a nullable value. Essentially, this
   *     is just n.getTypeI().isNullable(), but for purposes of this pass,
   *     the expression {@code x || null} is considered nullable even if
   *     x is always truthy. This often happens with expressions like
   *     {@code arr[i] || null}: The compiler doesn't know that arr[i] can
   *     be undefined.
   */
  private static boolean isNullable(Node n) {
    return n.getTypeI().isNullable()
        || (n.isOr() && n.getLastChild().isNull());
  }

  @Override
  public boolean shouldTraverse(NodeTraversal nodeTraversal, Node n, Node parent) {
    return true;
  }

  @Override
  public void process(Node externs, Node root) {
    NodeTraversal.traverseEs6(compiler, root, this);
  }

  @Override
  public void hotSwapScript(Node scriptRoot, Node originalRoot) {
    NodeTraversal.traverseEs6(compiler, originalRoot, this);
  }
}