TypeScript 5.4 New Features

TypeScript 5.4 Overview

TypeScript 5.4 was released in March 2024, adding features that improve developer experience, such as improved type inference and NoInfer type.

NoInfer Utility Type

NoInfer<T> is a utility type that prevents type parameter inference at specific positions.

Problematic Code (Before 5.4)

function createStreetLight<T extends string>(
    colors: T[],
    defaultColor: T
) {
    // ...
}

// 'blue' is also inferred
createStreetLight(['red', 'yellow', 'green'], 'blue');
// → T = 'red' | 'yellow' | 'green' | 'blue'

Solution Using NoInfer

function createStreetLight<T extends string>(
    colors: T[],
    defaultColor: NoInfer<T>
) {
    // ...
}

// Error! 'blue' is not included in 'red' | 'yellow' | 'green'
createStreetLight(['red', 'yellow', 'green'], 'blue');

Use Case: Useful when you want to infer types from only some arguments in a generic function.

Improved Type Narrowing in Closures

Type guards within closures have become smarter.

function getUrls(url: string | URL, names: string[]) {
    if (typeof url === 'string') {
        // Before: url remains string | URL
        // 5.4: url is narrowed to string
        return names.map(name => {
            return url + '/' + name;  // OK!
        });
    }
    return names.map(name => url.href + '/' + name);
}

Object.groupBy and Map.groupBy Support

Type definitions for new grouping methods have been added.

const users = [
    { name: 'Alice', age: 25 },
    { name: 'Bob', age: 30 },
    { name: 'Charlie', age: 25 }
];

// Object.groupBy
const byAge = Object.groupBy(users, user => user.age);
// { 25: [...], 30: [...] }

// Map.groupBy
const byAgeMap = Map.groupBy(users, user => user.age);
// Map { 25 => [...], 30 => [...] }

Import Attributes

You can specify attributes when importing modules such as JSON.

import config from './config.json' with { type: 'json' };

// Dynamic import
const data = await import('./data.json', {
    with: { type: 'json' }
});

Other Improvements

• require() type support improvement: Better ESM and CJS interoperability
• —moduleResolution bundler improvement: More accurate resolution in bundler environments
• Editor support improvements: Improved rename and import organization

Summary

TypeScript 5.4 enables more accurate and intentional typing with NoInfer type and closure type inference improvements. Object.groupBy type support is also a useful improvement for practical use.