Breaking Changes

Holger Jeromin edited this page Dec 27, 2016 · 80 revisions
Clone this wiki locally

These changes list where implementation differs between versions as the spec and compiler are simplified and inconsistencies are corrected.

For breaking changes to the compiler/services API, please check the API Breaking Changes page.

TypeScript 2.1

For full list of breaking changes see the breaking change issues.

Generated constructor code substitutes the return value of super(...) calls as this

In ES2015, constructors which return an object implicitly substitute the value of this for any callers of super(...). As a result, it is necessary to capture any potential return value of super(...) and replace it with this.

Example

A class C as:

class C extends B {
    public a: number;
    constructor() {
        super();
        this.a = 0;
    }
}

Will generate code as:

var C = (function (_super) {
    __extends(C, _super);
    function C() {
        var _this = _super.call(this) || this;
        _this.a = 0;
        return _this;
    }
    return C;
}(B));

Notice:

  • _super.call(this) is captured into a local variable _this
  • All uses of this in the constructor body has been replaced by the result of the super call (i.e. _this)
  • Each constructor now returns explicitly its this, to enable for correct inheritance

It is worth noting that the use of this before super(...) is already an error as of TypeScript 1.8

Extending built-ins like Error, Array, and Map may no longer work

As part of substituting the value of this with the value returned by a super(...) call, subclassing Error, Array, and others may no longer work as expected. This is due to the fact that constructor functions for Error, Array, and the like use ECMAScript 6's new.target to adjust the prototype chain; however, there is no way to ensure a value for new.target when invoking a constructor in ECMAScript 5. Other downlevel compilers generally have the same limitation by default.

Example

For a subclass like the following:

class FooError extends Error {
    constructor(m: string) {
        super(m);
    }
    sayHello() {
        return "hello " + this.message;
    }
}

you may find that:

  • methods may be undefined on objects returned by constructing these subclasses, so calling sayHello will result in an error.
  • instanceof will be broken between instances of the subclass and their instances, so (new FooError()) instanceof FooError will return false.

Recommendation

As a recommendation, you can manually adjust the prototype immediately after any super(...) calls.

class FooError extends Error {
    constructor(m: string) {
        super(m);

        // Set the prototype explicitly.
        Object.setPrototypeOf(this, FooError.prototype);
    }

    sayHello() {
        return "hello " + this.message;
    }
}

However, any subclass of FooError will have to manually set the prototype as well. For runtimes that don't support Object.setPrototypeOf, you may instead be able to use __proto__.

Unfortunately, these workarounds will not work on Internet Explorer 10 and prior. One can manually copy methods from the prototype onto the instance itself (i.e. FooError.prototype onto this), but the prototype chain itself cannot be fixed.

Literal types are inferred by default for const variables and readonly properties

String, numeric, boolean and enum literal types are not inferred by default for const declarations and readonly properties. This means your variables/properties an have more narrowed type than before. This could manifest in using comparison operators such as === and !===.

Example

const DEBUG = true; // Now has type `true`, previously had type `boolean`

if (DEBUG === false) { /// Error: operator '===' can not be applied to 'true' and 'false'
    ...
}

Recommendation

For types intentionally needed to be wider, cast to the base type:

const DEBUG = <boolean>true; // type is `boolean`

No type narrowing for captured variables in functions and class expressions

String, numeric and boolean literal types will be inferred if the generic type parameter has a constraint of string,number or boolean respectively. Moreover the rule of failing if no best common super-type for inferences in the case of literal types if they have the same base type (e.g. string).

Example

declare function push<T extends string>(...args: T[]): T;

var x = push("A", "B", "C"); // inferred as "A" | "B" | "C" in TS 2.1, was string in TS 2.0

Recommendation

Specify the type argument explicitly at call site:

var x = push<string>("A", "B", "C"); // x is string

Implicit-any error raised for un-annotated callback arguments with no matching overload arguments

Previously the compiler silently gave the argument of the callback (c below) a type any. The reason is how the compiler resolves function expressions while doing overload resolution.Starting with TypeScript 2.1 an error will be reported under --noImplicitAny.

Example

declare function func(callback: () => void): any;
declare function func(callback: (arg: number) => void): any;

func(c => { });

Recommendation

Remove the first overload, since it is rather meaningless; the function above can still be called with a call back with 1 or 0 required arguments, as it is safe for functions to ignore additional arguments.

declare function func(callback: (arg: number) => void): any;

func(c => { });
func(() => { });

Alternatively, you can either specify an explicit type annotation on the callback argument:

func((c:number) => { });

Comma operators on side-effect-free expressions is now flagged as an error

Mostly, this should catch errors that were previously allowed as valid comma expressions.

Example

let x = Math.pow((3, 5)); // x = NaN, was meant to be `Math.pow(3, 5)`

// This code does not do what it appears to!
let arr = [];
switch(arr.length) {
  case 0, 1:
    return 'zero or one';
  default:
    return 'more than one';
}

Recommendation

--allowUnreachableCode will disable the warning for the whole compilation. Alternatively, you can use the void operator to suppress the error for specific comma expressions:

let a = 0;
let y = (void a, 1); // no warning for `a`

Changes to DOM API's in the standard library

  • Node.firstChild, Node.lastChild, Node.nextSibling, Node.previousSibling, Node.parentElement and Node.parentNode are now Node | null instead of Node.

    See #11113 for more details.

    Recommendation is to explicitly check for null or use the ! assertion operator (e.g. node.lastChild!).

TypeScript 2.0

For full list of breaking changes see the breaking change issues.

No type narrowing for captured variables in functions and class expressions

Type narrowing does not cross function and class expressions, as well as lambda expressions.

Example

var x: number | string;

if (typeof x === "number") {
    function inner(): number {
        return x; // Error, type of x is not narrowed, c is number | string
    }
    var y: number = x; // OK, x is number
}

In the previous pattern the compiler can not tell when the callback will execute. Consider:

var x: number | string = "a";
if (typeof x === "string") {
    setTimeout(() => console.log(x.charAt(0)), 0);
}
x = 5;

It is wrong to assume x is a string when x.charAt() is called, as indeed it isn't.

Recommendation

Use constants instead:

const x: number | string = "a";
if (typeof x === "string") {
    setTimeout(() => console.log(x.charAt(0)), 0);
}

Generic type parameters are now narrowed

Example

function g<T>(obj: T) {
    var t: T;
    if (obj instanceof RegExp) {
         t = obj; // RegExp is not assignable to T
    }
}

Recommendation Either declare your locals to be a specific type and not the generic type parameter, or use a type assertion.

Getters with no setters are automatically inferred to be readonly properties

Example

class C {
  get x() { return 0; }
}

var c = new C();
c.x = 1; // Error Left-hand side is a readonly property

Recommendation

Define a setter or do not write to the property.

Function declarations not allowed in blocks in strict mode

This is already a run-time error under strict mode. Starting with TypeScript 2.0, it will be flagged as a compile-time error as well.

Example

if( true ) {
    function foo() {}
}

export = foo;

Recommendation

Use function expressions instead:

if( true ) {
    const foo = function() {}
}

TemplateStringsArray is now immutable

ES2015 tagged templates always pass their tag an immutable array-like object that has a property called raw (which is also immutable). TypeScript names this object the TemplateStringsArray.

Conveniently, TemplateStringsArray was assignable to an Array<string>, so it's possible users took advantage of this to use a shorter type for their tag parameters:

function myTemplateTag(strs: string[]) {
    // ...
}

However, in TypeScript 2.0, the language now supports the readonly modifier and can express that these objects are immutable. As a result, TemplateStringsArray has also been made immutable, and is no longer assignable to string[].

Recommendation

Use TemplateStringsArray explicitly (or use ReadonlyArray<string>).

TypeScript 1.8

For full list of breaking changes see the breaking change issues.

Modules are now emitted with a "use strict"; prologue

Modules were always parsed in strict mode as per ES6, but for non-ES6 targets this was not respected in the generated code. Starting with TypeScript 1.8, emitted modules are always in strict mode. This shouldn't have any visible changes in most code as TS considers most strict mode errors as errors at compile time, but it means that some things which used to silently fail at runtime in your TS code, like assigning to NaN, will now loudly fail. You can reference the MDN Article on strict mode for a detailed list of the differences between strict mode and non-strict mode.

To disable this behavior, pass --noImplicitUseStrict on the command line or set it in your tsconfig.json file.

Exporting non-local names from a module

In accordance with the ES6/ES2015 spec, it is an error to export a non-local name from a module.

Example

export { Promise }; // Error

Recommendation

Use a local variable declaration to capture the global name before exporting it.

const localPromise = Promise;
export { localPromise as Promise };

Reachability checks are enabled by default

In TypeScript 1.8 we've added a set of reachability checks to prevent certain categories of errors. Specifically

  1. check if code is reachable (enabled by default, can be disabled via allowUnreachableCode compiler option)

      function test1() {
          return 1;
          return 2; // error here
      }
    
      function test2(x) {
          if (x) {
              return 1;
          }
          else {
              throw new Error("NYI")
          }
          var y = 1; // error here
      }
  2. check if label is unused (enabled by default, can be disabled via allowUnusedLabels compiler option)

    l: // error will be reported - label `l` is unused
    while (true) {
    }
    
    (x) => { x:x } // error will be reported - label `x` is unused
  3. check if all code paths in function with return type annotation return some value (disabled by default, can be enabled via noImplicitReturns compiler option)

    // error will be reported since function does not return anything explicitly when `x` is falsy.
    function test(x): number {
      if (x) return 10;
    }
  4. check if control flow falls through cases in switch statement (disabled by default, can be enabled via noFallthroughCasesInSwitch compiler option). Note that cases without statements are not reported.

    switch(x) {
      // OK
      case 1:
      case 2:
          return 1;
    }
    switch(x) {
      case 1:
          if (y) return 1;
      case 2:
          return 2;
    }

If these errors are showing up in your code and you still think that scenario when they appear is legitimate you can suppress errors with compiler options.

--module is not allowed alongside --outFile unless --module is specified as one of amd or system.

Previously specifying both while using modules would result in an empty out file and no error.

Changes to DOM API's in the standard library

  • ImageData.data is now of type Uint8ClampedArray instead of number[]. See #949 for more details.
  • HTMLSelectElement .options is now of type HTMLCollection instead of HTMLSelectElement. See #1558 for more details.
  • HTMLTableElement.createCaption, HTMLTableElement.createTBody, HTMLTableElement.createTFoot, HTMLTableElement.createTHead, HTMLTableElement.insertRow, HTMLTableSectionElement.insertRow, and HTMLTableElement.insertRow now return HTMLTableRowElement instead of HTMLElement. See #3583 for more details.
  • HTMLTableRowElement.insertCell now return HTMLTableCellElement instead of HTMLElement. See #3583 for more details.
  • IDBObjectStore.createIndex and IDBDatabase.createIndex second argument is now of type IDBObjectStoreParameters instead of any. See #5932 for more details.
  • DataTransferItemList.Item returns type now is DataTransferItem instead of File. See #6106 for more details.
  • Window.open return type now is Window instead of any. See #6418 for more details.
  • WeakMap.clear as removed. See #6500 for more details.

Disallow this accessing before super-call

ES6 disallows accessing this in a constructor declaration.

For example:

class B {
    constructor(that?: any) {}
}

class C extends B {
    constructor() {
        super(this);  // error;
    }
}

class D extends B {
    private _prop1: number;
    constructor() {
        this._prop1 = 10;  // error
        super();
    }
}

TypeScript 1.7

For full list of breaking changes see the breaking change issues.

Changes in inferring the type from this

In a class, the type of the value this will be inferred to the this type. This means subsequent assignments from values the original type can fail.

Example:

class Fighter {
    /** @returns the winner of the fight. */
    fight(opponent: Fighter) {
        let theVeryBest = this;
        if (Math.rand() < 0.5) {
            theVeryBest = opponent; // error
        }
        return theVeryBest
    }
}

Recommendations:

Add a type annotation:

class Fighter {
    /** @returns the winner of the fight. */
    fight(opponent: Fighter) {
        let theVeryBest: Fighter = this;
        if (Math.rand() < 0.5) {
            theVeryBest = opponent; // no error
        }
        return theVeryBest
    }
}

Automatic semicolon insertion after class member modifiers

The keywords abstract, public, protected and private are FutureReservedWords in ECMAScript 3 and are subject to automatic semicolon insertion. Previously, TypeScript did not insert semicolons when these keywords were on their own line. Now that this is fixed, abstract class D no longer correctly extends C in the following example, and instead declares a concrete method m and an additional property named abstract.

Note that async and declare already correctly did ASI.

Example:

abstract class C {
    abstract m(): number;
}
abstract class D extends C {
    abstract
    m(): number;
}

Recommendations:

Remove line breaks after keywords when defining class members. In general, avoid relying on automatic semicolon insertion.

TypeScript 1.6

For full list of breaking changes see the breaking change issues.

Strict object literal assignment checking

It is an error to specify properties in an object literal that were not specified on the target type, when assigned to a variable or passed for a parameter of a non-empty target type.

This new strictness can be disabled with the --suppressExcessPropertyErrors compiler option.

Example:

var x: { foo: number };
x = { foo: 1, baz: 2 };  // Error, excess property `baz`

var y: { foo: number, bar?: number };
y = { foo: 1, baz: 2 };  // Error, excess or misspelled property `baz`

Recommendations:

To avoid the error, there are few remedies based on the situation you are looking into:

If the target type accepts additional properties, add an indexer:

var x: { foo: number, [x: string]: any };
x = { foo: 1, baz: 2 };  // OK, `baz` matched by index signature

If the source types are a set of related types, explicitly specify them using union types instead of just specifying the base type.

let animalList: (Dog | Cat | Turkey)[] = [    // use union type instead of Animal
    {name: "Milo", meow: true },
    {name: "Pepper", bark: true},
    {name: "koko", gobble: true}
];

Otherwise, explicitly cast to the target type to avoid the warning message:

interface Foo {
    foo: number;
}
interface FooBar {
    foo: number;
    bar: number;
}
var y: Foo;
y = <FooBar>{ foo: 1, bar: 2 };

CommonJS module resolution no longer assumes paths are relative

Previously, for the files one.ts and two.ts, an import of "one" in two.ts would resolve to one.ts if they resided in the same directory.

In TypeScript 1.6, "one" is no longer equivalent to "./one" when compiling with CommonJS. Instead, it is searched as relative to an appropriate node_modules folder as would be resolved by runtimes such as Node.js. For details, see the issue that describes the resolution algorithm.

Example:

./one.ts

export function f() {
    return 10;
}

./two.ts

import { f as g } from "one";

Recommendations:

Fix any non-relative import names that were unintended (strongly suggested).

./one.ts

export function f() {
    return 10;
}

./two.ts

import { f as g } from "./one";

Set the --moduleResolution compiler option to classic.

Function and class default export declarations can no longer merge with entities intersecting in their meaning

Declaring an entity with the same name and in the same space as a default export declaration is now an error; for example,

export default function foo() {
}

namespace foo {
    var x = 100;
}

and

export default class Foo {
    a: number;
}

interface Foo {
    b: string;
}

both cause an error.

However, in the following example, merging is allowed because the namespace does does not have a meaning in the value space:

export default class Foo {
}

namespace Foo {
}

Recommendations:

Declare a local for your default export and use a separate export default statement as so:

class Foo {
    a: number;
}

interface foo {
    b: string;
}

export default Foo;

For more details see the originating issue.

Module bodies are parsed in strict mode

In accordance with the ES6 spec, module bodies are now parsed in strict mode. module bodies will behave as if "use strict" was defined at the top of their scope; this includes flagging the use of arguments and eval as variable or parameter names, use of future reserved words as variables or parameters, use of octal numeric literals, etc..

Changes to DOM API's in the standard library

  • MessageEvent and ProgressEvent constructors now expect arguments; see issue #4295 for more details.
  • ImageData constructor now expects arguments; see issue #4220 for more details.
  • File constructor now expects arguments; see issue #3999 for more details.

System module output uses bulk exports

The compiler uses the new bulk-export variation of the _export function in the System module format that takes any object containing key value pairs (optionally an entire module object for export *) as arguments instead of key, value.

The module loader needs to be updated to v0.17.1 or higher.

.js content of npm package is moved from 'bin' to 'lib' folder

Entry point of TypeScript npm package was moved from bin to lib to unblock scenarios when 'node_modules/typescript/bin/typescript.js' is served from IIS (by default bin is in the list of hidden segments so IIS will block access to this folder).

TypeScript npm package does not install globally by default

TypeScript 1.6 removes the preferGlobal flag from package.json. If you rely on this behaviour please use npm install -g typescript.

Decorators are checked as call expressions

Starting with 1.6, decorators type checking is more accurate; the compiler will checks a decorator expression as a call expression with the decorated entity as a parameter. This can cause error to be reported that were not in previous releases.

TypeScript 1.5

For full list of breaking changes see the breaking change issues.

Referencing arguments in arrow functions is not allowed

This is an alignment with the ES6 semantics of arrow functions. Previously arguments within an arrow function would bind to the arrow function arguments. As per ES6 spec draft 9.2.12, arrow functions do not have an arguments objects. In TypeScript 1.5, the use of arguments object in arrow functions will be flagged as an error to ensure your code ports to ES6 with no change in semantics.

Example:

function f() {
    return () => arguments; // Error: The 'arguments' object cannot be referenced in an arrow function.
}

Recommendations:

// 1. Use named rest args
function f() {
    return (...args) => { args; }
}

// 2. Use function expressions instead
function f() {
    return function(){ arguments; }
}

Enum reference in-lining changes

For regular enums, pre 1.5, the compiler only inline constant members, and a member was only constant if its initializer was a literal. That resulted in inconsistent behavior depending on whether the enum value is initalized with a literal or an expression. Starting with Typescript 1.5 all non-const enum members are not inlined.

Example:

var x = E.a;  // previously inlined as "var x = 1; /*E.a*/"

enum E {
   a = 1
}

Recommendation: Add the const modifier to the enum declaration to ensure it is consistently inlined at all consumption sites.

For more details see issue #2183.

Contextual type flows through super and parenthesized expressions

Prior to this release, contextual types did not flow through parenthesized expressions. This has forced explicit type casts, especially in cases where parentheses are required to make an expression parse.

In the examples below, m will have a contextual type, where previously it did not.

var x: SomeType = (n) => ((m) => q);
var y: SomeType = t ? (m => m.length) : undefined;

class C extends CBase<string> {
    constructor() {
        super({
            method(m) { return m.length; }
        });
    }
}

See issues #1425 and #920 for more details.

DOM interface changes

TypeScript 1.5 refreshes the DOM types in lib.d.ts. This is the first major refresh since TypeScript 1.0; many IE-specific definitions have been removed in favor of the standard DOM definitions, as well as adding missing types like Web Audio and touch events.

Workaround:

You can keep using older versions of the library with newer version of the compiler. You will need to include a local copy of a previous version in your project. Here is the last released version before this change (TypeScript 1.5-alpha).

Here is a list of changes:

  • Property selection is removed from type Document
  • Property clipboardData is removed from type Window
  • Removed interface MSEventAttachmentTarget
  • Properties onresize, disabled, uniqueID, removeNode, fireEvent, currentStyle, runtimeStyle are removed from type HTMLElement
  • Property url is removed from type Event
  • Properties execScript, navigate, item are removed from type Window
  • Properties documentMode, parentWindow, createEventObject are removed from type Document
  • Property parentWindow is removed from type HTMLDocument
  • Property setCapture does not exist anywhere now
  • Property releaseCapture does not exist anywhere now
  • Properties setAttribute, styleFloat, pixelLeft are removed from type CSSStyleDeclaration
  • Property selectorText is removed from type CSSRule
  • CSSStyleSheet.rules is of type CSSRuleList instead of MSCSSRuleList
  • documentElement is of type Element instead of HTMLElement
  • Event has a new required property returnValue
  • Node has a new required property baseURI
  • Element has a new required property classList
  • Location has a new required property origin
  • Properties MSPOINTER_TYPE_MOUSE, MSPOINTER_TYPE_TOUCH are removed from type MSPointerEvent
  • CSSStyleRule has a new required property readonly
  • Property execUnsafeLocalFunction is removed from type MSApp
  • Global method toStaticHTML is removed
  • HTMLCanvasElement.getContext now returns CanvasRenderingContext2D | WebGLRenderingContex
  • Removed extension types Dataview, Weakmap, Map, Set
  • XMLHttpRequest.send has two overloads send(data?: Document): void; and send(data?: String): void;
  • window.orientation is of type string instead of number
  • IE-specific attachEvent and detachEvent are removed from Window

Here is a list of libraries that are partly or entirely replaced by the added DOM types:

  • DefinitelyTyped/auth0/auth0.d.ts
  • DefinitelyTyped/gamepad/gamepad.d.ts
  • DefinitelyTyped/interactjs/interact.d.ts
  • DefinitelyTyped/webaudioapi/waa.d.ts
  • DefinitelyTyped/webcrypto/WebCrypto.d.ts

For more details, please see the full change.

Class bodies are parsed in strict mode

In accordance with the ES6 spec, class bodies are now parsed in strict mode. Class bodies will behave as if "use strict" was defined at the top of their scope; this includes flagging the use of arguments and eval as variable or parameter names, use of future reserved words as variables or parameters, use of octal numeric literals, etc..

TypeScript 1.4

For full list of breaking changes see the breaking change issues.

See issue #868 for more details about breaking changes related to Union Types

Multiple Best Common Type Candidates

Given multiple viable candidates from a Best Common Type computation we now choose an item (depending on the compiler's implementation) rather than the first item.

var a: { x: number; y?: number };
var b: { x: number; z?: number };

// was { x: number; z?: number; }[]
// now { x: number; y?: number; }[]
var bs = [b, a];

This can happen in a variety of circumstances. A shared set of required properties and a disjoint set of other properties (optional or otherwise), empty types, compatible signature types (including generic and non-generic signatures when type parameters are stamped out with any).

Recommendation Provide a type annotation if you need a specific type to be chosen

var bs: { x: number; y?: number; z?: number }[] = [b, a];

Generic Type Inference

Using different types for multiple arguments of type T is now an error, even with constraints involved:

declare function foo<T>(x: T, y:T): T;
var r = foo(1, ""); // r used to be {}, now this is an error

With constraints:

interface Animal { x }
interface Giraffe extends Animal { y }
interface Elephant extends Animal { z }
function f<T extends Animal>(x: T, y: T): T { return undefined; }
var g: Giraffe;
var e: Elephant;
f(g, e);

See https://github.com/Microsoft/TypeScript/pull/824#discussion_r18665727 for explanation.

Recommendations Specify an explicit type parameter if the mismatch was intentional:

var r = foo<{}>(1, ""); // Emulates 1.0 behavior
var r = foo<string|number>(1, ""); // Most useful
var r = foo<any>(1, ""); // Easiest
f<Animal>(g, e);

or rewrite the function definition to specify that mismatches are OK:

declare function foo<T,U>(x: T, y:U): T|U;
function f<T extends Animal, U extends Animal>(x: T, y: U): T|U { return undefined; }

Generic Rest Parameters

You cannot use heterogeneous argument types anymore:

function makeArray<T>(...items: T[]): T[] { return items; }
var r = makeArray(1, ""); // used to return {}[], now an error

Likewise for new Array(...)

Recommendations Declare a back-compat signature if the 1.0 behavior was desired:

function makeArray<T>(...items: T[]): T[];
function makeArray(...items: {}[]): {}[];
function makeArray<T>(...items: T[]): T[] { return items; }

Overload Resolution with Type Argument Inference

var f10: <T>(x: T, b: () => (a: T) => void, y: T) => T;
var r9 = f10('', () => (a => a.foo), 1); // r9 was any, now this is an error

Recommendations Manually specify a type parameter

var r9 = f10<any>('', () => (a => a.foo), 1);

Strict Mode Parsing for Class Declarations and Class Expressions

ECMAScript 2015 Language Specification (ECMA-262 6th Edition) specifies that ClassDeclaration and ClassExpression are strict mode productions. Thus, additional restrictions will be applied when parsing a class declaration or class expression.

Examples:

class implements {}  // Invalid: implements is a reserved word in strict mode
class C {
    foo(arguments: any) {   // Invalid: "arguments" is not allow as a function argument
        var eval = 10;      // Invalid: "eval" is not allowed as the left-hand-side expression
        arguments = [];     // Invalid: arguments object is immutable
    }
}

For complete list of strict mode restrictions, please see Annex C - The Strict Mode of ECMAScript of ECMA-262 6th Edition.

TypeScript 1.1

For full list of breaking changes see the breaking change issues.

Working with null and undefined in ways that are observably incorrect is now an error

Examples:

var ResultIsNumber17 = +(null + undefined);
// Operator '+' cannot be applied to types 'undefined' and 'undefined'.

var ResultIsNumber18 = +(null + null);
// Operator '+' cannot be applied to types 'null' and 'null'.

var ResultIsNumber19 = +(undefined + undefined);
// Operator '+' cannot be applied to types 'undefined' and 'undefined'.

Similarly, using null and undefined directly as objects that have methods now is an error

Examples:

null.toBAZ();

undefined.toBAZ();