Unleashing the Power of TypeScript/

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Inferring Generics

Inferring Generics

Learn how TypeScript simplifies generic type inference.

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TypeScript is always trying to get out of our way. If it can figure out what a generic ought to be, it will relieve us of the responsibility of telling it. Let’s take a look at the following code:

TypeScript 4.9.5
export const identity = <T>(x: T): T => {
  return x;
};
export const foo = identity<string>('foo');
export const two = identity(2);
console.log(foo);
console.log(two);

In the code above, we’re able to completely omit telling TypeScript what type T should be. It seems that we’re handing that function call a number, so it’s able to figure out that T is a number in this case.

We can also leverage this to simplify our linked list above:

TypeScript 4.9.5
type LinkedListNode<T> = {
  value: T;
  next?: LinkedListNode<T>;
};
const createLinkedListNode = <T>(value: T): LinkedListNode<T> => { 
    return { value };
};
const firstNode = createLinkedListNode('first');
const secondNode = createLinkedListNode('second');
firstNode.next = secondNode;
console.log(firstNode);

Let’s explain the code sample above:

  • Lines 6–8: We define a generic function named createLinkedListNode that receive a value as a parameter, and return the linked list node with that value.

  • Lines 10–12: We create two linked list nodes and name them firstNode and secondNode by calling the createLinkedListNode ...