What is contextual typing?

Q: What is contextual typing?

const names = ["Alice", "Bob", "Eve"]; // Contextual typing for function - parameter s inferred to have type string names.forEach(function (s) { console.log(s.toUpperCase()); }); // Contextual typing also applies to arrow functions names.forEach((s) => { console.log(s.toUpperCase()); });

Difficulty: 5/10

type inference, function signatures, generic inference

Contextual typing is when TypeScript infers a type from the context/location where an expression appears — rather than from the expression itself. TypeScript looks at where something is used to figure out what type it should be.

javascript

Scenario Questions

0-2 years experience

1You have `const add = (a, b) => a + b;` and you assign it to a variable typed as `(x: number, y: number) => number`. What does TypeScript infer for the parameters, and why?
2If you write `let handler: (e: MouseEvent) => void = e => console.log(e.button);` and later call `handler({})`, what error appears and how does contextual typing cause it?

2-5 years experience

Follow-up Questions