Default generic types in Typescript - Step-by-Step Execution

Concept Flow - Default generic types

Define generic type with default

↓

Use generic without specifying type?

Yes→Use default type

No↓

Use generic with specified type

↓

Function or class runs with chosen type

This flow shows how a generic type with a default is defined and used either by specifying a type or by falling back to the default.

Execution Sample

Typescript

function wrap<T = string>(value?: T): (T | undefined)[] {
  return [value];
}

const a = wrap(123);
const b = wrap('hello');
const c = wrap();

This code defines a generic function with a default type string and calls it with different arguments to show default and explicit generic usage.

Execution Table

Step	Call	Generic Type Used	Value Passed	Return Value
1	wrap(123)	number (inferred)	123	[123]
2	wrap('hello')	string (inferred)	'hello'	['hello']
3	wrap()	string (default)	undefined	[undefined]

💡 All calls complete; default generic type used when no argument is passed.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
a	undefined	[123]	[123]	[123]
b	undefined	undefined	['hello']	['hello']
c	undefined	undefined	undefined	[undefined]

Key Moments - 3 Insights

Why does wrap() use string as the generic type even though no argument is passed?

Because the generic type T has a default of string, when no type or argument is provided, TypeScript uses string as the default type (see execution_table step 3).

How does TypeScript decide the generic type when we call wrap(123)?

TypeScript infers the generic type from the argument's type, so T becomes number for wrap(123) (see execution_table step 1).

What happens if we explicitly specify a generic type when calling wrap?

The specified type overrides the default and inference, so the function uses that type for T.

Visual Quiz - 3 Questions

Look at the execution table, what generic type is used in step 2 when calling wrap('hello')?

A string (default)

B string (inferred)

C number (default)

D any

💡 Hint

Check the 'Generic Type Used' column in execution_table row for step 2.

At which step does the function use the default generic type?

A Step 3

B Step 2

C Step 1

D None

💡 Hint

Look for 'string (default)' in the 'Generic Type Used' column in execution_table.

If we call wrap<boolean>(true), how would the generic type and return value appear?

A Generic type: string, Return: ['true']

B Generic type: any, Return: [true]

C Generic type: boolean, Return: [true]

D Generic type: boolean, Return: true

💡 Hint

Explicit generic types override defaults and inference; return is an array of the passed value.

Concept Snapshot

Default generic types let you set a fallback type for generics.
Syntax: function fn<T = DefaultType>(arg: T) { ... }
If no type is given or inferred, default is used.
Explicit types override default and inference.
Useful for flexible, safe code with sensible defaults.

Full Transcript

This lesson shows how default generic types work in TypeScript. We define a generic function with a default type string. When calling wrap with a number or string argument, TypeScript infers the generic type from the argument. When calling wrap with no argument, the default generic type string is used. Variables a, b, and c track the returned arrays. Key points include how inference and defaults interact, and how explicit generic types override both. The quizzes check understanding of these steps.