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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 1.6

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

.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).

Module body is 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..

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.

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.

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, either define a indexer on the target type or use type assertion on the object literal in question.

var x: { foo: number, [x: string]: any };
x = { foo: 1, baz: 2 };  // Ok, `baz` matched by index signature
interface Foo {
    foo: number;
}
interface Bar extends Foo {
    bar: number;
}
var y: Foo;
y = <Bar>{ foo: 1, bar: 2 };

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.

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 body is 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 microsoft/TypeScript#824 (comment) 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();