How to use union and intersection types

Short answer

Union types (A | B) allow a value to be one of several types. Intersection types (A & B) require a value to satisfy all combined types.

Steps

1. Declare a union: type ID = string | number;
2. Narrow a union using typeof, in, or custom type guards before accessing type-specific properties.
3. Declare an intersection: type Employee = Person & HasId;
4. Use intersections to mix multiple object shapes into one.
5. Combine unions and intersections for advanced modeling.

Tips

• Discriminated unions with a shared literal property make narrowing straightforward and type-safe.
• Intersections can produce never if primitive types conflict, such as string & number.

Common issues

• Accessing a property that exists on only one member of a union causes a compile error without narrowing.
• Overusing intersections can create bloated types that are hard to read and maintain.