Improved control flow analysis in TypeScript 5.5

11 June 2024

TypeScript 5.5 ships with some nice ergonomic improvements to type inference when working with arrays and indexed accessed objects.

Isn't it annoying when the TypeScript compiler can't infer a type that you as developer felt was quite obvious to deduce? I know that I hate to write code just to please the compiler. This has thankfully improved a lot of the years and has now been improved further in TypeScript 5.5 (pending release as of writing)

Inferred Type Predicates

Type inference has now been improved when working with arrays:

const items = [1, 2, "three"];
.filter(item => typeof item === "string")
.map(item =>
// item: number | string

Previously, if you tried to compile the above code, the compiler would tell you Property 'toUpperCase' does not exist on type 'number'. To fix this you would have to throw in an additional type check inside the map function:

const items = [1, 2, "three"];
.filter(item => typeof item === "string")
.map(item => {
// Checking for string again
if (typeof item ==== "string") {
return item.toUpperCase();

lo and behold I was pleasantly surprised the other day when I by accident tried to compile some code without the extra type check and it somehow worked! Only then did I learn that TypeScript 5.5 can Infer type predicates from function bodies using control flow analysis. Which means:

const items = [1, 2, "three"];
.filter(item => typeof item === "string")
.map(item =>
// item: string

Here's another similar example of what we can now do without having to resort to extra type checks just to satisfy the compiler!

type Cat = {
type: "cat";
type Dog = {
type: "dog";
color: string;
type Animal = Cat | Dog;
let animals: Animal[] = [
{ type: "cat" },
{ type: "dog", color: "pink" },
{ type: "dog", color: "red" }
let dogColors = animals
.filter(animal => animal.type === "dog")
.map(dog => dog.color);
console.log(dogColors); // => ["pink", "red"]

Try it out yourself in the TypeScript Playground. Fun fact: The same developer that raised this issue 7 years ago now finally managed to fix it!

Control Flow Narrowing for Constant Indexed Accesses

Another improvement is that TypeScript is now able to narrow expressions of the form obj[key] when both ´obj´ and ´key´ are constant.

function test(obj: Record<string, unknown>, key: string) {
if (typeof obj[key] === "string") {
obj[key].toUpperCase(); // Now ok, previously was "Object is of type 'unknown'".
greeting: "hello"

Trying to compile the above code prior to 5.5 would fail, but now works as expected. Try out the example yourself in the TypeScript Playground.

That's it. Less screaming, kicking and fighting the compiler, more fun and games. Enjoy!

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Hi, thanks for reading!

I’m Daniel, Software Engineer from Sweden. If you have any questions regarding this article please reach out to me on Twitter. You can also find me on GitHub.

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