v8/test/mjsunit/mjsunit.js

// Copyright 2008 the V8 project authors. All rights reserved.
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
//     * Redistributions of source code must retain the above copyright
//       notice, this list of conditions and the following disclaimer.
//     * Redistributions in binary form must reproduce the above
//       copyright notice, this list of conditions and the following
//       disclaimer in the documentation and/or other materials provided
//       with the distribution.
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//       contributors may be used to endorse or promote products derived
//       from this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

function MjsUnitAssertionError(message) {
  this.message = message;
  // Temporarily install a custom stack trace formatter and restore the
  // previous value.
  let prevPrepareStackTrace = Error.prepareStackTrace;
  try {
    Error.prepareStackTrace = MjsUnitAssertionError.prepareStackTrace;
    // This allows fetching the stack trace using TryCatch::StackTrace.
    this.stack = new Error("MjsUnitAssertionError").stack;
  } finally {
    Error.prepareStackTrace = prevPrepareStackTrace;
  }
}

/*
 * This file is included in all mini jsunit test cases.  The test
 * framework expects lines that signal failed tests to start with
 * the f-word and ignore all other lines.
 */

MjsUnitAssertionError.prototype.toString = function () {
  return this.message + "\n\nStack: " + this.stack;
};


// Expected and found values the same objects, or the same primitive
// values.
// For known primitive values, please use assertEquals.
var assertSame;

// Inverse of assertSame.
var assertNotSame;

// Expected and found values are identical primitive values or functions
// or similarly structured objects (checking internal properties
// of, e.g., Number and Date objects, the elements of arrays
// and the properties of non-Array objects).
var assertEquals;

// Deep equality predicate used by assertEquals.
var deepEquals;

// Expected and found values are not identical primitive values or functions
// or similarly structured objects (checking internal properties
// of, e.g., Number and Date objects, the elements of arrays
// and the properties of non-Array objects).
var assertNotEquals;

// The difference between expected and found value is within certain tolerance.
var assertEqualsDelta;

// The found object is an Array with the same length and elements
// as the expected object. The expected object doesn't need to be an Array,
// as long as it's "array-ish".
var assertArrayEquals;

// The found object must have the same enumerable properties as the
// expected object. The type of object isn't checked.
var assertPropertiesEqual;

// Assert that the string conversion of the found value is equal to
// the expected string. Only kept for backwards compatibility, please
// check the real structure of the found value.
var assertToStringEquals;

// Checks that the found value is true. Use with boolean expressions
// for tests that doesn't have their own assertXXX function.
var assertTrue;

// Checks that the found value is false.
var assertFalse;

// Checks that the found value is null. Kept for historical compatibility,
// please just use assertEquals(null, expected).
var assertNull;

// Checks that the found value is *not* null.
var assertNotNull;

// Assert that the passed function or eval code throws an exception.
// The optional second argument is an exception constructor that the
// thrown exception is checked against with "instanceof".
// The optional third argument is a message type string or RegExp object that is
// compared to the message of the thrown exception.
var assertThrows;

// Assert that the passed function throws an exception.
// The exception is checked against the second argument using assertEquals.
var assertThrowsEquals;

// Assert that the passed promise does not resolve, but eventually throws an
// exception. The optional second argument is an exception constructor that the
// thrown exception is checked against with "instanceof".
// The optional third argument is a message type string or RegExp object that is
// compared to the message of the thrown exception.
var assertThrowsAsync;

// Assert that the passed function or eval code does not throw an exception.
var assertDoesNotThrow;

// Asserts that the found value is an instance of the constructor passed
// as the second argument.
var assertInstanceof;

// Assert that this code is never executed (i.e., always fails if executed).
var assertUnreachable;

// Assert that the function code is (not) optimized.  If "no sync" is passed
// as second argument, we do not wait for the concurrent optimization thread to
// finish when polling for optimization status.
// Only works with --allow-natives-syntax.
var assertOptimized;
var assertUnoptimized;

// Assert that a string contains another expected substring.
var assertContains;

// Assert that a string matches a given regex.
var assertMatches;

// Assert that a promise resolves or rejects.
// Parameters:
// {promise} - the promise
// {success} - optional - a callback which is called with the result of the
//             resolving promise.
//  {fail} -   optional - a callback which is called with the result of the
//             rejecting promise. If the promise is rejected but no {fail}
//             callback is set, the error is propagated out of the promise
//             chain.
var assertPromiseResult;

var promiseTestChain;
var promiseTestCount = 0;

// These bits must be in sync with bits defined in Runtime_GetOptimizationStatus
var V8OptimizationStatus = {
  kIsFunction: 1 << 0,
  kNeverOptimize: 1 << 1,
  kAlwaysOptimize: 1 << 2,
  kMaybeDeopted: 1 << 3,
  kOptimized: 1 << 4,
  kTurboFanned: 1 << 5,
  kInterpreted: 1 << 6,
  kMarkedForOptimization: 1 << 7,
  kMarkedForConcurrentOptimization: 1 << 8,
  kOptimizingConcurrently: 1 << 9,
  kIsExecuting: 1 << 10,
  kTopmostFrameIsTurboFanned: 1 << 11,
  kLiteMode: 1 << 12,
};

// Returns true if --lite-mode is on and we can't ever turn on optimization.
var isNeverOptimizeLiteMode;

// Returns true if --no-opt mode is on.
var isNeverOptimize;

// Returns true if --always-opt mode is on.
var isAlwaysOptimize;

// Returns true if given function in interpreted.
var isInterpreted;

// Returns true if given function is optimized.
var isOptimized;

// Returns true if given function is compiled by TurboFan.
var isTurboFanned;

// Monkey-patchable all-purpose failure handler.
var failWithMessage;

// Returns the formatted failure text.  Used by test-async.js.
var formatFailureText;

// Returns a pretty-printed string representation of the passed value.
var prettyPrinted;

(function () {  // Scope for utility functions.

  var ObjectPrototypeToString = Object.prototype.toString;
  var NumberPrototypeValueOf = Number.prototype.valueOf;
  var BooleanPrototypeValueOf = Boolean.prototype.valueOf;
  var StringPrototypeValueOf = String.prototype.valueOf;
  var DatePrototypeValueOf = Date.prototype.valueOf;
  var RegExpPrototypeToString = RegExp.prototype.toString;
  var ArrayPrototypeForEach = Array.prototype.forEach;
  var ArrayPrototypeJoin = Array.prototype.join;
  var ArrayPrototypeMap = Array.prototype.map;
  var ArrayPrototypePush = Array.prototype.push;

  var BigIntPrototypeValueOf;
  // TODO(neis): Remove try-catch once BigInts are enabled by default.
  try {
    BigIntPrototypeValueOf = BigInt.prototype.valueOf;
  } catch (e) {}

  function classOf(object) {
    // Argument must not be null or undefined.
    var string = ObjectPrototypeToString.call(object);
    // String has format [object <ClassName>].
    return string.substring(8, string.length - 1);
  }


  function ValueOf(value) {
    switch (classOf(value)) {
      case "Number":
        return NumberPrototypeValueOf.call(value);
      case "BigInt":
        return BigIntPrototypeValueOf.call(value);
      case "String":
        return StringPrototypeValueOf.call(value);
      case "Boolean":
        return BooleanPrototypeValueOf.call(value);
      case "Date":
        return DatePrototypeValueOf.call(value);
      default:
        return value;
    }
  }


  prettyPrinted = function prettyPrinted(value) {
    switch (typeof value) {
      case "string":
        return JSON.stringify(value);
      case "bigint":
        return String(value) + "n";
      case "number":
        if (value === 0 && (1 / value) < 0) return "-0";
        // FALLTHROUGH.
      case "boolean":
      case "undefined":
      case "function":
      case "symbol":
        return String(value);
      case "object":
        if (value === null) return "null";
        var objectClass = classOf(value);
        switch (objectClass) {
          case "Number":
          case "BigInt":
          case "String":
          case "Boolean":
          case "Date":
            return objectClass + "(" + prettyPrinted(ValueOf(value)) + ")";
          case "RegExp":
            return RegExpPrototypeToString.call(value);
          case "Array":
            var mapped = ArrayPrototypeMap.call(
                value, prettyPrintedArrayElement);
            var joined = ArrayPrototypeJoin.call(mapped, ",");
            return "[" + joined + "]";
          case "Uint8Array":
          case "Int8Array":
          case "Int16Array":
          case "Uint16Array":
          case "Uint32Array":
          case "Int32Array":
          case "Float32Array":
          case "Float64Array":
            var joined = ArrayPrototypeJoin.call(value, ",");
            return objectClass + "([" + joined + "])";
          case "Object":
            break;
          default:
            return objectClass + "(" + String(value) + ")";
        }
        // [[Class]] is "Object".
        var name = value.constructor.name;
        if (name) return name + "()";
        return "Object()";
      default:
        return "-- unknown value --";
    }
  }


  function prettyPrintedArrayElement(value, index, array) {
    if (value === undefined && !(index in array)) return "";
    return prettyPrinted(value);
  }


  failWithMessage = function failWithMessage(message) {
    throw new MjsUnitAssertionError(message);
  }

  formatFailureText = function(expectedText, found, name_opt) {
    var message = "Fail" + "ure";
    if (name_opt) {
      // Fix this when we ditch the old test runner.
      message += " (" + name_opt + ")";
    }

    var foundText = prettyPrinted(found);
    if (expectedText.length <= 40 && foundText.length <= 40) {
      message += ": expected <" + expectedText + "> found <" + foundText + ">";
    } else {
      message += ":\nexpected:\n" + expectedText + "\nfound:\n" + foundText;
    }
    return message;
  }

  function fail(expectedText, found, name_opt) {
    return failWithMessage(formatFailureText(expectedText, found, name_opt));
  }


  function deepObjectEquals(a, b) {
    var aProps = Object.keys(a);
    aProps.sort();
    var bProps = Object.keys(b);
    bProps.sort();
    if (!deepEquals(aProps, bProps)) {
      return false;
    }
    for (var i = 0; i < aProps.length; i++) {
      if (!deepEquals(a[aProps[i]], b[aProps[i]])) {
        return false;
      }
    }
    return true;
  }


  deepEquals = function deepEquals(a, b) {
    if (a === b) {
      // Check for -0.
      if (a === 0) return (1 / a) === (1 / b);
      return true;
    }
    if (typeof a !== typeof b) return false;
    if (typeof a === "number") return isNaN(a) && isNaN(b);
    if (typeof a !== "object" && typeof a !== "function") return false;
    // Neither a nor b is primitive.
    var objectClass = classOf(a);
    if (objectClass !== classOf(b)) return false;
    if (objectClass === "RegExp") {
      // For RegExp, just compare pattern and flags using its toString.
      return RegExpPrototypeToString.call(a) ===
             RegExpPrototypeToString.call(b);
    }
    // Functions are only identical to themselves.
    if (objectClass === "Function") return false;
    if (objectClass === "Array") {
      var elementCount = 0;
      if (a.length !== b.length) {
        return false;
      }
      for (var i = 0; i < a.length; i++) {
        if (!deepEquals(a[i], b[i])) return false;
      }
      return true;
    }
    if (objectClass === "String" || objectClass === "Number" ||
      objectClass === "BigInt" || objectClass === "Boolean" ||
      objectClass === "Date") {
      if (ValueOf(a) !== ValueOf(b)) return false;
    }
    return deepObjectEquals(a, b);
  }

  assertSame = function assertSame(expected, found, name_opt) {
    // TODO(mstarzinger): We should think about using Harmony's egal operator
    // or the function equivalent Object.is() here.
    if (found === expected) {
      if (expected !== 0 || (1 / expected) === (1 / found)) return;
    } else if ((expected !== expected) && (found !== found)) {
      return;
    }
    fail(prettyPrinted(expected), found, name_opt);
  };

  assertNotSame = function assertNotSame(expected, found, name_opt) {
    // TODO(mstarzinger): We should think about using Harmony's egal operator
    // or the function equivalent Object.is() here.
    if (found !== expected) {
      if (expected === 0 || (1 / expected) !== (1 / found)) return;
    } else if (!((expected !== expected) && (found !== found))) {
      return;
    }
    fail(prettyPrinted(expected), found, name_opt);
  }

  assertEquals = function assertEquals(expected, found, name_opt) {
    if (!deepEquals(found, expected)) {
      fail(prettyPrinted(expected), found, name_opt);
    }
  };

  assertNotEquals = function assertNotEquals(expected, found, name_opt) {
    if (deepEquals(found, expected)) {
      fail("not equals to " + prettyPrinted(expected), found, name_opt);
    }
  };


  assertEqualsDelta =
      function assertEqualsDelta(expected, found, delta, name_opt) {
    if (Math.abs(expected - found) > delta) {
      fail(prettyPrinted(expected) + " +- " + prettyPrinted(delta), found, name_opt);
    }
  };


  assertArrayEquals = function assertArrayEquals(expected, found, name_opt) {
    var start = "";
    if (name_opt) {
      start = name_opt + " - ";
    }
    assertEquals(expected.length, found.length, start + "array length");
    if (expected.length === found.length) {
      for (var i = 0; i < expected.length; ++i) {
        assertEquals(expected[i], found[i],
                     start + "array element at index " + i);
      }
    }
  };


  assertPropertiesEqual = function assertPropertiesEqual(expected, found,
                                                         name_opt) {
    // Check properties only.
    if (!deepObjectEquals(expected, found)) {
      fail(expected, found, name_opt);
    }
  };


  assertToStringEquals = function assertToStringEquals(expected, found,
                                                       name_opt) {
    if (expected !== String(found)) {
      fail(expected, found, name_opt);
    }
  };


  assertTrue = function assertTrue(value, name_opt) {
    assertEquals(true, value, name_opt);
  };


  assertFalse = function assertFalse(value, name_opt) {
    assertEquals(false, value, name_opt);
  };


  assertNull = function assertNull(value, name_opt) {
    if (value !== null) {
      fail("null", value, name_opt);
    }
  };


  assertNotNull = function assertNotNull(value, name_opt) {
    if (value === null) {
      fail("not null", value, name_opt);
    }
  };

  function executeCode(code) {
    if (typeof code === 'function')  return code();
    if (typeof code === 'string') return eval(code);
    failWithMessage(
        'Given code is neither function nor string, but ' + (typeof code) +
        ': <' + prettyPrinted(code) + '>');
  }

  function checkException(e, type_opt, cause_opt) {
    if (type_opt !== undefined) {
      assertEquals('function', typeof type_opt);
      assertInstanceof(e, type_opt);
    }
    if (RegExp !== undefined && cause_opt instanceof RegExp) {
      assertMatches(cause_opt, e.message, 'Error message');
    } else if (cause_opt !== undefined) {
      assertEquals(cause_opt, e.message, 'Error message');
    }
  }

  assertThrows = function assertThrows(code, type_opt, cause_opt) {
    if (arguments.length > 1 && type_opt === undefined) {
      failWithMessage('invalid use of assertThrows, unknown type_opt given');
    }
    if (type_opt !== undefined && typeof type_opt !== 'function') {
      failWithMessage(
          'invalid use of assertThrows, maybe you want assertThrowsEquals');
    }
    try {
      executeCode(code);
    } catch (e) {
      checkException(e, type_opt, cause_opt);
      return;
    }
    let msg = 'Did not throw exception';
    if (type_opt !== undefined && type_opt.name !== undefined)
      msg += ', expected ' + type_opt.name;
    failWithMessage(msg);
  };

  assertThrowsEquals = function assertThrowsEquals(fun, val) {
    try {
      fun();
    } catch (e) {
      assertSame(val, e);
      return;
    }
    failWithMessage('Did not throw exception, expected ' + prettyPrinted(val));
  };

  assertThrowsAsync = function assertThrowsAsync(promise, type_opt, cause_opt) {
    if (arguments.length > 1 && type_opt === undefined) {
      failWithMessage('invalid use of assertThrows, unknown type_opt given');
    }
    if (type_opt !== undefined && typeof type_opt !== 'function') {
      failWithMessage(
          'invalid use of assertThrows, maybe you want assertThrowsEquals');
    }
    let msg = 'Promise did not throw exception';
    if (type_opt !== undefined && type_opt.name !== undefined)
      msg += ', expected ' + type_opt.name;
    return assertPromiseResult(
        promise,
        // Use setTimeout to throw the error again to get out of the promise
        // chain.
        res => setTimeout(_ => fail('<throw>', res, msg), 0),
        e => checkException(e, type_opt, cause_opt));
  };

  assertInstanceof = function assertInstanceof(obj, type) {
    if (!(obj instanceof type)) {
      var actualTypeName = null;
      var actualConstructor = Object.getPrototypeOf(obj).constructor;
      if (typeof actualConstructor === "function") {
        actualTypeName = actualConstructor.name || String(actualConstructor);
      }
      failWithMessage("Object <" + prettyPrinted(obj) + "> is not an instance of <" +
               (type.name || type) + ">" +
               (actualTypeName ? " but of <" + actualTypeName + ">" : ""));
    }
  };

  assertDoesNotThrow = function assertDoesNotThrow(code, name_opt) {
    try {
      executeCode(code);
    } catch (e) {
      if (e instanceof MjsUnitAssertionError) throw e;
      failWithMessage("threw an exception: " + (e.message || e));
    }
  };

  assertUnreachable = function assertUnreachable(name_opt) {
    // Fix this when we ditch the old test runner.
    var message = "Fail" + "ure: unreachable";
    if (name_opt) {
      message += " - " + name_opt;
    }
    failWithMessage(message);
  };

  assertContains = function(sub, value, name_opt) {
    if (value == null ? (sub != null) : value.indexOf(sub) == -1) {
      fail("contains '" + String(sub) + "'", value, name_opt);
    }
  };

  assertMatches = function(regexp, str, name_opt) {
    if (!(regexp instanceof RegExp)) {
      regexp = new RegExp(regexp);
    }
    if (!str.match(regexp)) {
      fail("should match '" + regexp + "'", str, name_opt);
    }
  };

  function concatenateErrors(stack, exception) {
    // If the exception does not contain a stack trace, wrap it in a new Error.
    if (!exception.stack) exception = new Error(exception);

    // If the exception already provides a special stack trace, we do not modify
    // it.
    if (typeof exception.stack !== 'string') {
      return exception;
    }
    exception.stack = stack + '\n\n' + exception.stack;
    return exception;
  }

  assertPromiseResult = function(promise, success, fail) {
    if (success !== undefined) assertEquals('function', typeof success);
    if (fail !== undefined) assertEquals('function', typeof fail);
    assertInstanceof(promise, Promise);
    const stack = (new Error()).stack;

    var test_promise = promise.then(
        result => {
          try {
            if (--promiseTestCount == 0) testRunner.notifyDone();
            if (success !== undefined) success(result);
          } catch (e) {
            // Use setTimeout to throw the error again to get out of the promise
            // chain.
            setTimeout(_ => {
              throw concatenateErrors(stack, e);
            }, 0);
          }
        },
        result => {
          try {
            if (--promiseTestCount == 0) testRunner.notifyDone();
            if (fail === undefined) throw result;
            fail(result);
          } catch (e) {
            // Use setTimeout to throw the error again to get out of the promise
            // chain.
            setTimeout(_ => {
              throw concatenateErrors(stack, e);
            }, 0);
          }
        });

    if (!promiseTestChain) promiseTestChain = Promise.resolve();
    // waitUntilDone is idempotent.
    testRunner.waitUntilDone();
    ++promiseTestCount;
    return promiseTestChain.then(test_promise);
  };

  var OptimizationStatusImpl = undefined;

  var OptimizationStatus = function(fun, sync_opt) {
    if (OptimizationStatusImpl === undefined) {
      try {
        OptimizationStatusImpl = new Function(
            "fun", "sync", "return %GetOptimizationStatus(fun, sync);");
      } catch (e) {
        throw new Error("natives syntax not allowed");
      }
    }
    return OptimizationStatusImpl(fun, sync_opt);
  }

  assertUnoptimized = function assertUnoptimized(
      fun, sync_opt, name_opt, skip_if_maybe_deopted = true) {
    if (sync_opt === undefined) sync_opt = "";
    var opt_status = OptimizationStatus(fun, sync_opt);
    // Tests that use assertUnoptimized() do not make sense if --always-opt
    // option is provided. Such tests must add --no-always-opt to flags comment.
    assertFalse((opt_status & V8OptimizationStatus.kAlwaysOptimize) !== 0,
                "test does not make sense with --always-opt");
    assertTrue((opt_status & V8OptimizationStatus.kIsFunction) !== 0, name_opt);
    if (skip_if_maybe_deopted &&
        (opt_status & V8OptimizationStatus.kMaybeDeopted) !== 0) {
      // When --deopt-every-n-times flag is specified it's no longer guaranteed
      // that particular function is still deoptimized, so keep running the test
      // to stress test the deoptimizer.
      return;
    }
    assertFalse((opt_status & V8OptimizationStatus.kOptimized) !== 0, name_opt);
  }

  assertOptimized = function assertOptimized(
      fun, sync_opt, name_opt, skip_if_maybe_deopted = true) {
    if (sync_opt === undefined) sync_opt = "";
    var opt_status = OptimizationStatus(fun, sync_opt);
    // Tests that use assertOptimized() do not make sense for Lite mode where
    // optimization is always disabled, explicitly exit the test with a warning.
    if (opt_status & V8OptimizationStatus.kLiteMode) {
      print("Warning: Test uses assertOptimized in Lite mode, skipping test.");
      testRunner.quit(0);
    }
    // Tests that use assertOptimized() do not make sense if --no-opt
    // option is provided. Such tests must add --opt to flags comment.
    assertFalse((opt_status & V8OptimizationStatus.kNeverOptimize) !== 0,
                "test does not make sense with --no-opt");
    assertTrue(
        (opt_status & V8OptimizationStatus.kIsFunction) !== 0,
        'should be a function: ' + name_opt);
    if (skip_if_maybe_deopted &&
        (opt_status & V8OptimizationStatus.kMaybeDeopted) !== 0) {
      // When --deopt-every-n-times flag is specified it's no longer guaranteed
      // that particular function is still optimized, so keep running the test
      // to stress test the deoptimizer.
      return;
    }
    assertTrue(
        (opt_status & V8OptimizationStatus.kOptimized) !== 0,
        'should be optimized: ' + name_opt);
  }

  isNeverOptimizeLiteMode = function isNeverOptimizeLiteMode() {
    var opt_status = OptimizationStatus(undefined, "");
    return (opt_status & V8OptimizationStatus.kLiteMode) !== 0;
  }

  isNeverOptimize = function isNeverOptimize() {
    var opt_status = OptimizationStatus(undefined, "");
    return (opt_status & V8OptimizationStatus.kNeverOptimize) !== 0;
  }

  isAlwaysOptimize = function isAlwaysOptimize() {
    var opt_status = OptimizationStatus(undefined, "");
    return (opt_status & V8OptimizationStatus.kAlwaysOptimize) !== 0;
  }

  isInterpreted = function isInterpreted(fun) {
    var opt_status = OptimizationStatus(fun, "");
    assertTrue((opt_status & V8OptimizationStatus.kIsFunction) !== 0,
               "not a function");
    return (opt_status & V8OptimizationStatus.kOptimized) === 0 &&
           (opt_status & V8OptimizationStatus.kInterpreted) !== 0;
  }

  isOptimized = function isOptimized(fun) {
    var opt_status = OptimizationStatus(fun, "");
    assertTrue((opt_status & V8OptimizationStatus.kIsFunction) !== 0,
               "not a function");
    return (opt_status & V8OptimizationStatus.kOptimized) !== 0;
  }

  isTurboFanned = function isTurboFanned(fun) {
    var opt_status = OptimizationStatus(fun, "");
    assertTrue((opt_status & V8OptimizationStatus.kIsFunction) !== 0,
               "not a function");
    return (opt_status & V8OptimizationStatus.kOptimized) !== 0 &&
           (opt_status & V8OptimizationStatus.kTurboFanned) !== 0;
  }

  // Custom V8-specific stack trace formatter that is temporarily installed on
  // the Error object.
  MjsUnitAssertionError.prepareStackTrace = function(error, stack) {
    // Trigger default formatting with recursion.
    try {
      // Filter-out all but the first mjsunit frame.
      let filteredStack = [];
      let inMjsunit = true;
      for (let i = 0; i < stack.length; i++) {
        let frame = stack[i];
        if (inMjsunit) {
          let file = frame.getFileName();
          if (!file || !file.endsWith("mjsunit.js")) {
            inMjsunit = false;
            // Push the last mjsunit frame, typically containing the assertion
            // function.
            if (i > 0) ArrayPrototypePush.call(filteredStack, stack[i-1]);
            ArrayPrototypePush.call(filteredStack, stack[i]);
          }
          continue;
        }
        ArrayPrototypePush.call(filteredStack, frame);
      }
      stack = filteredStack;

      // Infer function names and calculate {max_name_length}
      let max_name_length = 0;
      ArrayPrototypeForEach.call(stack, each => {
        let name = each.getFunctionName();
        if (name == null) name = "";
        if (each.isEval()) {
          name = name;
        } else if (each.isConstructor()) {
          name = "new " + name;
        } else if (each.isNative()) {
          name = "native " + name;
        } else if (!each.isToplevel()) {
          name = each.getTypeName() + "." + name;
        }
        each.name = name;
        max_name_length = Math.max(name.length, max_name_length)
      });

      // Format stack frames.
      stack = ArrayPrototypeMap.call(stack, each => {
        let frame = "    at " + each.name.padEnd(max_name_length);
        let fileName = each.getFileName();
        if (each.isEval()) return frame + " " + each.getEvalOrigin();
        frame += " " + (fileName ? fileName : "");
        let line= each.getLineNumber();
        frame += " " + (line ? line : "");
        let column = each.getColumnNumber();
        frame += (column ? ":" + column : "");
        return frame;
      });
      return "" + error.message + "\n" + ArrayPrototypeJoin.call(stack, "\n");
    } catch (e) {};
    return error.stack;
  }
})();