Intersection type syntax in Typescript - Step-by-Step Execution

Concept Flow - Intersection type syntax

Define Type A

↓

Define Type B

↓

Use & to combine

↓

Create variable of combined type

↓

Access properties from both A and B

↓

Use variable safely with all properties

Intersection types combine multiple types into one that has all properties from each type.

Execution Sample

Typescript

type A = { name: string };
type B = { age: number };

const person: A & B = { name: "Alice", age: 30 };

console.log(person.name, person.age);

This code creates a variable with properties from both types A and B using intersection.

Execution Table

Step	Action	Evaluation	Result
1	Define type A	A = { name: string }	Type A created
2	Define type B	B = { age: number }	Type B created
3	Create variable person with type A & B	person = { name: "Alice", age: 30 }	person has name and age
4	Access person.name	person.name	"Alice"
5	Access person.age	person.age	30
6	Print person.name and person.age	console.log(person.name, person.age)	Output: Alice 30

💡 All steps complete, variable person has combined properties from A and B.

Variable Tracker

Variable	Start	After 3	Final
person	undefined	{ name: "Alice", age: 30 }	{ name: "Alice", age: 30 }

Key Moments - 2 Insights

Why can person have both name and age properties?

Because person is typed as A & B, it must have all properties from both types, as shown in execution_table step 3.

What happens if person misses one property from A or B?

TypeScript will give an error because intersection requires all properties, so missing any property breaks the type, as implied by step 3.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what properties does person have?

A Only name

B Both name and age

C Only age

D Neither name nor age

💡 Hint

Check the 'Result' column in step 3 of execution_table.

At which step do we see the output printed to the console?

A Step 4

B Step 5

C Step 6

D Step 2

💡 Hint

Look for console.log action in execution_table.

If we remove age from person in step 3, what will happen?

A TypeScript error because age is missing

B No error, code runs fine

C person will have age as undefined

D person will only have name property

💡 Hint

Refer to key_moments about missing properties in intersection types.

Concept Snapshot

Intersection types combine multiple types using &.
Syntax: type Combined = Type1 & Type2;
Variables must have all properties from each type.
Useful to merge object shapes safely.
TypeScript checks all properties exist.

Full Transcript

This visual trace shows how intersection types work in TypeScript. First, two types A and B are defined with different properties. Then, a variable person is created with type A & B, meaning it must have all properties from both types. The variable is assigned an object with name and age. Accessing person.name and person.age works because both exist. The console.log prints both values. If any property is missing, TypeScript will give an error. This ensures variables have combined properties safely.