Type inference with generics

TypeScriptJunior~1 min read

What type inference means in the context of generics

When you call a function with a generic parameter, TypeScript analyzes the arguments passed and substitutes the type itself for T (or other parameters). That is, you don't need to explicitly write <string>, <number>, and so on - the compiler does this automatically.

Example 1: basic type inference

javascript

function identity<T>(value: T): T {
  return value;
}

const a = identity("Hello"); // T = string
const b = identity(42);      // T = number

TypeScript sees that the argument "Hello" has type string, and substitutes T = string automatically.

Example 2: inference from multiple arguments

javascript

function merge<T, U>(a: T, b: U): T & U {
  return Object.assign({}, a, b);
}

const result = merge({ name: "Tim" }, { age: 25 });
// TypeScript infers T = { name: string }, U = { age: number }
// The result has type { name: string; age: number }

Here T and U are inferred separately for each argument. TypeScript combines them via &.

Example 3: inference from context (contextual inference)

javascript

function wrap<T>(value: T): { data: T } {
  return { data: value };
}

const wrapped = wrap({ id: 1, name: "Tim" });
// T = { id: number; name: string }

TypeScript "sees" the structure of the passed object and infers the exact type.

Example 4: partial inference

javascript

function pair<T, U = string>(first: T, second?: U): [T, U] {
  return [first, second as U];
}

const p1 = pair(1);           // T = number, U = string (default)
const p2 = pair(true, false); // T = boolean, U = boolean

You can set a default type for a generic parameter - it is then used when TypeScript cannot infer the type from the arguments.

Example 5: type inference on a function's return value

javascript

function makeArray<T>(item: T, times: number): T[] {
  return Array(times).fill(item);
}

const arr = makeArray(5, 3); // T = number -> number[]

The type T is inferred from the first argument, item.

Example 6: when inference does not work

javascript

function logValue<T>(value: T): void {
  console.log(value);
}

logValue(null);  // T cannot be inferred precisely
logValue(undefined); // the same applies

In such cases the type must be specified explicitly:

javascript

logValue<string | null>(null);

How inference works under the hood

TypeScript applies several strategies:

From arguments to type parameters - analyzing the arguments and their types.
From return type - when the result type can be computed.
Contextual typing - analyzing the context in which the function is used.

An example of contextual inference:

javascript

const handler = <T>(value: T) => console.log(value);

["a", "b", "c"].forEach(handler);
// TypeScript understands that T = string

Summary

What TS does	Example	Result
Infers the type from an argument	`identity(42)`	`T = number`
Infers the type from multiple arguments	`merge({a: 1}, {b: 2})`	`T = {a: number}, U = {b: number}`
Applies the default type	`pair(1)`	`U = string`
Contextual inference	`["x"].forEach(handler)`	`T = string`
Cannot infer the type	`logValue(null)`	must be specified explicitly

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