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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 2016 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.common.base.Preconditions;
import com.google.common.collect.ImmutableMap;
import com.google.javascript.rhino.IR;
import com.google.javascript.rhino.Node;
import java.util.HashMap;
import java.util.Map;

/**
 * Rewrites a JSON file to be a module export. So that the JSON file parses correctly, it is wrapped
 * in an EXPR_RESULT. The pass makes only basic checks that the file provided is valid JSON. It is
 * not a full JSON validator.
 *
 * Looks for JSON files named "package.json" so that the "main" property can be used as an alias
 * in module name resolution. See https://docs.npmjs.com/files/package.json#main
 */
public class RewriteJsonToModule extends NodeTraversal.AbstractPostOrderCallback
    implements CompilerPass {
  public static final DiagnosticType JSON_UNEXPECTED_TOKEN =
      DiagnosticType.error("JSC_JSON_UNEXPECTED_TOKEN", "Unexpected JSON token");

  private final Map packageJsonMainEntries;
  private final Compiler compiler;

  /**
   * Creates a new RewriteJsonToModule instance which can be used to rewrite JSON files to modules.
   *
   * @param compiler The compiler
   */
  public RewriteJsonToModule(Compiler compiler) {
    this.compiler = compiler;
    this.packageJsonMainEntries = new HashMap<>();
  }

  public ImmutableMap getPackageJsonMainEntries() {
    return ImmutableMap.copyOf(packageJsonMainEntries);
  }

  /**
   * Module rewriting is done a on per-file basis prior to main compilation. The root node for each
   * file is a SCRIPT - not the typical jsRoot of other passes.
   */
  @Override
  public void process(Node externs, Node root) {
    Preconditions.checkState(root.isScript());
    NodeTraversal.traverseEs6(compiler, root, this);
  }

  @Override
  public void visit(NodeTraversal t, Node n, Node parent) {
    switch (n.getToken()) {
      case SCRIPT:
        if (n.getChildCount() != 1) {
          compiler.report(t.makeError(n, JSON_UNEXPECTED_TOKEN));
        } else {
          visitScript(t, n, parent);
        }
        return;

      case OBJECTLIT:
      case ARRAYLIT:
      case NUMBER:
      case TRUE:
      case FALSE:
      case NULL:
      case STRING:
        break;

      case STRING_KEY:
        if (!n.isQuotedString() || !n.hasOneChild()) {
          compiler.report(t.makeError(n, JSON_UNEXPECTED_TOKEN));
        }
        break;

      case EXPR_RESULT:
        if (!parent.isScript()) {
          compiler.report(t.makeError(n, JSON_UNEXPECTED_TOKEN));
        }
        break;

      default:
        compiler.report(t.makeError(n, JSON_UNEXPECTED_TOKEN));
        break;
    }

    if (n.getLineno() == 1) {
      // We wrapped the expression in parens so our first-line columns are off by one.
      // We need to correct for this.
      n.setCharno(n.getCharno() - 1);
      compiler.reportCodeChange();
    }
  }

  /**
   * For script nodes of JSON objects, add a module variable assignment so the result is exported.
   *
   * If the file path ends with "/package.json", look for a "main" key in the object literal and
   * track it as a module alias.
   */
  private void visitScript(NodeTraversal t, Node n, Node parent) {
    if (n.getChildCount() != 1 || !n.getFirstChild().isExprResult()) {
      compiler.report(t.makeError(n, JSON_UNEXPECTED_TOKEN));
      return;
    }

    Node jsonObject = n.getFirstFirstChild().detach();
    n.removeFirstChild();

    String moduleName = t.getInput().getPath().toModuleName();

    n.addChildToFront(
        IR.var(IR.name(moduleName).useSourceInfoFrom(jsonObject), jsonObject)
            .useSourceInfoFrom(jsonObject));

    n.addChildToFront(
        IR.exprResult(
                IR.call(IR.getprop(IR.name("goog"), IR.string("provide")), IR.string(moduleName)))
            .useSourceInfoIfMissingFromForTree(n));

    String inputPath = t.getInput().getSourceFile().getOriginalPath();
    if (inputPath.endsWith("/package.json") && jsonObject.isObjectLit()) {
      Node main = NodeUtil.getFirstPropMatchingKey(jsonObject, "main");
      if (main != null && main.isString()) {
        String dirName = inputPath.substring(0, inputPath.length() - "package.json".length());
        packageJsonMainEntries.put(inputPath, dirName + main.getString());
      }
    }

    compiler.reportCodeChange();
  }
}