Strict function types

TypeScript 2.6 introduces a new strict checking flag, --strictFunctionTypes. The --strictFunctionTypes switch is part of the --strict family of switches, meaning that it defaults to on in --strict mode. You can opt-out by setting --strictFunctionTypes false on your command line or in your tsconfig.json.

Under --strictFunctionTypes function type parameter positions are checked contravariantly instead of bivariantly. For some background on what variance means for function types check out What are covariance and contravariance?.

The stricter checking applies to all function types, except those originating in method or constructor declarations. Methods are excluded specifically to ensure generic classes and interfaces (such as Array<T>) continue to mostly relate covariantly.

Consider the following example in which Animal is the supertype of Dog and Cat:

declare let f1: (x: Animal) => void;
declare let f2: (x: Dog) => void;
declare let f3: (x: Cat) => void;
f1 = f2;  // Error with --strictFunctionTypes
f2 = f1;  // Ok
f2 = f3;  // Error

The first assignment is permitted in default type checking mode, but flagged as an error in strict function types mode. Intuitively, the default mode permits the assignment because it is possibly sound, whereas strict function types mode makes it an error because it isn’t provably sound. In either mode the third assignment is an error because it is never sound.

Another way to describe the example is that the type (x: T) => void is bivariant (i.e. covariant or contravariant) for T in default type checking mode, but contravariant for T in strict function types mode.

interface Comparer<T> {
    compare: (a: T, b: T) => number;

declare let animalComparer: Comparer<Animal>;
declare let dogComparer: Comparer<Dog>;

animalComparer = dogComparer;  // Error
dogComparer = animalComparer;  // Ok

The first assignment is now an error. Effectively, T is contravariant in Comparer<T> because it is used only in function type parameter positions.

By the way, note that whereas some languages (e.g. C# and Scala) require variance annotations (out/in or +/-), variance emerges naturally from the actual use of a type parameter within a generic type due to TypeScript’s structural type system.


Under --strictFunctionTypes the first assignment is still permitted if compare was declared as a method. Effectively, T is bivariant in Comparer<T> because it is used only in method parameter positions.

interface Comparer<T> {
    compare(a: T, b: T): number;

declare let animalComparer: Comparer<Animal>;
declare let dogComparer: Comparer<Dog>;

animalComparer = dogComparer;  // Ok because of bivariance
dogComparer = animalComparer;  // Ok

TypeScript 2.6 also improves type inference involving contravariant positions:

function combine<T>(...funcs: ((x: T) => void)[]): (x: T) => void {
    return x => {
        for (const f of funcs) f(x);

function animalFunc(x: Animal) {}
function dogFunc(x: Dog) {}

let combined = combine(animalFunc, dogFunc);  // (x: Dog) => void

Above, all inferences for T originate in contravariant positions, and we therefore infer the best common subtype for T. This contrasts with inferences from covariant positions, where we infer the best common supertype.

Cache tagged template objects in modules

TypeScript 2.6 fixes the tagged string template emit to align better with the ECMAScript spec. As per the ECMAScript spec, every time a template tag is evaluated, the same template strings object (the same TemplateStringsArray) should be passed as the first argument. Before TypeScript 2.6, the generated output was a completely new template object each time. Though the string contents are the same, this emit affects libraries that use the identity of the string for cache invalidation purposes, e.g. lit-html.

export function id(x: TemplateStringsArray) {
    return x;

export function templateObjectFactory() {
    return id`hello world`;

let result = templateObjectFactory() === templateObjectFactory(); // true in TS 2.6

Results in the following generated code:

"use strict";
var __makeTemplateObject = (this && this.__makeTemplateObject) || function (cooked, raw) {
    if (Object.defineProperty) { Object.defineProperty(cooked, "raw", { value: raw }); } else { cooked.raw = raw; }
    return cooked;

function id(x) {
    return x;

var _a;
function templateObjectFactory() {
    return id(_a || (_a = __makeTemplateObject(["hello world"], ["hello world"])));

var result = templateObjectFactory() === templateObjectFactory();

Note: This change brings a new emit helper, __makeTemplateObject; if you are using --importHelpers with tslib, an updated to version 1.8 or later.

Localized diagnostics on the command line

TypeScript 2.6 npm package ships with localized versions of diagnostic messages for 13 languages. The localized messages are available when using --locale flag on the command line.


Error messages in Russian:

c:\ts>tsc --v
Version 2.6.0-dev.20171003

c:\ts>tsc --locale ru --pretty c:\test\a.ts

../test/a.ts(1,5): error TS2322: Тип ""string"" не может быть назначен для типа "number".

1 var x: number = "string";

And help in Japanese:

PS C:\ts> tsc --v
Version 2.6.0-dev.20171003

PS C:\ts> tsc --locale ja-jp
バージョン 2.6.0-dev.20171003
構文: tsc [オプション] [ファイル ...]

例:  tsc hello.ts
    tsc --outFile file.js file.ts
    tsc @args.txt

 -h, --help                                 このメッセージを表示します。
 --all                                      コンパイラ オプションをすべて表示します。
 -v, --version                              コンパイラのバージョンを表示します。
 --init                                     TypeScript プロジェクトを初期化して、tsconfig.json ファイルを作成します。
 -p ファイルまたはディレクトリ, --project ファイルまたはディレクトリ  構成ファイルか、'tsconfig.json' を含むフォルダーにパスが指定されたプロジェクトをコ
 --pretty                                   色とコンテキストを使用してエラーとメッセージにスタイルを適用します (試験的)。
 -w, --watch                                入力ファイルを監視します。
 -t バージョン, --target バージョン                   ECMAScript のターゲット バージョンを指定します: 'ES3' (既定)、'ES5'、'ES2015'、'ES2016'、'ES2017'、'ES
 -m 種類, --module 種類                         モジュール コード生成を指定します: 'none'、'commonjs'、'amd'、'system'、'umd'、'es2015'、'ESNext'。
 --lib                                      コンパイルに含めるライブラリ ファイルを指定します:
                                              'es5' 'es6' 'es2015' 'es7' 'es2016' 'es2017' 'esnext' 'dom' 'dom.iterable' 'webworker' 'scripthost' 'es201
5.core' 'es2015.collection' 'es2015.generator' 'es2015.iterable' 'es2015.promise' 'es2015.proxy' 'es2015.reflect' 'es2015.symbol' 'es2015.symbol.wellkno
wn' 'es2016.array.include' 'es2017.object' 'es2017.sharedmemory' 'es2017.string' 'es2017.intl' 'esnext.asynciterable'
 --allowJs                                  javascript ファイルのコンパイルを許可します。
 --jsx 種類                                   JSX コード生成を指定します: 'preserve'、'react-native'、'react'。
 -d, --declaration                          対応する '.d.ts' ファイルを生成します。
 --sourceMap                                対応する '.map' ファイルを生成します。
 --outFile ファイル                             出力を連結して 1 つのファイルを生成します。
 --outDir ディレクトリ                            ディレクトリへ出力構造をリダイレクトします。
 --removeComments                           コメントを出力しないでください。
 --noEmit                                   出力しないでください。
 --strict                                   strict 型チェックのオプションをすべて有効にします。
 --noImplicitAny                            暗黙的な 'any' 型を含む式と宣言に関するエラーを発生させます。
 --strictNullChecks                         厳格な null チェックを有効にします。
 --noImplicitThis                           暗黙的な 'any' 型を持つ 'this' 式でエラーが発生します。
 --alwaysStrict                             厳格モードで解析してソース ファイルごとに "use strict" を生成します。
 --noUnusedLocals                           使用されていないローカルに関するエラーを報告します。
 --noUnusedParameters                       使用されていないパラメーターに関するエラーを報告します。
 --noImplicitReturns                        関数の一部のコード パスが値を返さない場合にエラーを報告します。
 --noFallthroughCasesInSwitch               switch ステートメントに case のフォールスルーがある場合にエラーを報告します。
 --types                                    コンパイルに含む型宣言ファイル。

Suppress errors in .ts files using ‘// @ts-ignore’ comments

TypeScript 2.6 support suppressing errors in .js files using // @ts-ignore comments placed above the offending lines.

if (false) {
    // @ts-ignore: Unreachable code error

A // @ts-ignore comment suppresses all errors that originate on the following line. It is recommended practice to have the remainder of the comment following @ts-ignore explain which error is being suppressed.

Please note that this comment only suppresses the error reporting, and we recommend you use this comments very sparingly.

Faster tsc --watch

TypeScript 2.6 brings a faster --watch implementation. The new version optimizes code generation and checking for code bases using ES modules. Changes detected in a module file will result in only regenerating the changed module, and files that depend on it, instead of the whole project. Projects with large number of files should reap the most benefit from this change.

The new implementation also brings performance enhancements to watching in tsserver. The watcher logic has been completely rewritten to respond faster to change events.

Write-only references now flagged as unused

TypeScript 2.6 adds revised implementation the --noUnusedLocals and --noUnusedParameters compiler options. Declarations are only written to but never read from are now flagged as unused.


Bellow both n and m will be marked as unused, because their values are never read. Previously TypeScript would only check whether their values were referenced.

function f(n: number) {
    n = 0;

class C {
    private m: number;
    constructor() {
        this.m = 0;

Also functions that are only called within their own bodies are considered unused.

function f() {
    f(); // Error: 'f' is declared but its value is never read