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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 2020 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.type;

import com.google.javascript.jscomp.base.Tri;

/**
 * An enum for representing boolean outcomes. Currently they just represent typical booleans but
 * this enum will be expanded to specifically differentiate between falsy and explicitly nullish
 * values. e.g. 0 is falsy but not nullish.
 */
public enum Outcome {
  /** Represents truthy values. For example: {}, true, 1, etc. */
  TRUE {
    @Override
    public boolean isTruthy() {
      return true;
    }

    @Override
    public Tri isNullish() {
      return Tri.FALSE;
    }

    @Override
    public Outcome not() {
      return Outcome.FALSE;
    }
  },

  /** Represents falsy values. For examples: '', 0, false, null, etc. */
  FALSE {
    @Override
    public boolean isTruthy() {
      return false;
    }

    @Override
    public Tri isNullish() {
      return Tri.UNKNOWN;
    }

    @Override
    public Outcome not() {
      return Outcome.TRUE;
    }
  };

  /** Determines whether an Outcome enum value is truthy. */
  public abstract boolean isTruthy();

  /**
   * Determines whether an Outcome enum value is nullish. Using Tri instead of a boolean because 0
   * is Outcome.FALSE but not nullish so sometimes it is unclear.
   */
  public abstract Tri isNullish();

  /** Gets the {@code not} of {@code this}. */
  public abstract Outcome not();

  /** Gets the Outcome for the given boolean. */
  public static Outcome forBoolean(boolean val) {
    return val ? Outcome.TRUE : Outcome.FALSE;
  }
}