Overview of New Features in TypeScript 4.6

New Feature Overview

TypeScript 4.6 adds a set of language improvements, including the ability to run constructor code before the super() call, enhanced control‑flow analysis for destructured discriminated unions and dependent parameters, recursion‑depth checks, and indexed‑access inference optimizations.

Support for executing code before super()

This feature allows code that does not reference this to run before the mandatory super() call in derived class constructors, overcoming a previous limitation of the TypeScript implementation.

Example

class Base {}

class Derived extends Base {
    someProperty = true;

    constructor() {
        // error! must call super() first because it needs to initialize someProperty.
        doSomeStuff();
        super();
    }
}

New Performance Analysis Tool: TypeScript Trace Analyzer

The compiler now provides a --generateTrace flag that produces a detailed timing report of the compilation process, helping developers diagnose performance bottlenecks, although the raw output may be hard to read without the dedicated visualizer.

ES2022 Target Support

The --target es2022 option is now available, enabling stable output for features such as class fields and new built‑in methods like Array.at() , Object.hasOwn() , and the Error.cause option.

Further reading: ECMAScript 2022 preview

More Syntax Checks for JavaScript Files

TypeScript now performs additional syntax validation on plain JavaScript files, reporting errors such as duplicate declarations, illegal modifiers on export statements, and multiple default cases in a switch . The @ts-nocheck comment can be used to disable type checking for a file.

Examples

const foo = 1234;
//    ~~~
// error: Cannot redeclare block‑scoped variable 'foo'.

const foo = 5678;
//    ~~~
// error: Cannot redeclare block‑scoped variable 'foo'.

function container() {
    export function foo() {
        //  ~~~~~~
        // error: Modifiers cannot appear here.
    }
}

Breaking Changes

Some existing behaviors have been altered; developers should review the migration guide when upgrading to TypeScript 4.6.