In TypeScript, an intersection does not let the right-hand type override the left. A & B requires a value to satisfy both types, so conflicting property declarations can make that property never. To intentionally change a property, remove it from the original type with Omit before adding its replacement.
Why an intersection creates a conflict
An intersection combines requirements: a value of type A & B must satisfy both A and B. If both types declare the same property with incompatible types, TypeScript combines those property requirements rather than choosing one. The result can be a property of type never, meaning no ordinary value can satisfy it. See the TypeScript Handbook’s explanation of object intersections.
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type Person1 = { name: string };
type Person2 = { name: number };
type Staff = Person1 & Person2;
// Staff.name must be both string and number.
// No ordinary value can satisfy both, so the property is never.When a property becomes unusable, ask whether the intended value should really satisfy both constraints. If it should not, the intersection likely represents the wrong relationship.
Replace a property with Omit
When a derived type should keep the rest of a base shape but deliberately change one property, exclude that key first and then add the new declaration. TypeScript’s utility types reference documents Omit for constructing a type while excluding selected keys.
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type ApiRecord = {
id: string;
value: number;
};
type DisplayRecord = Omit<ApiRecord, "value"> & {
value: string;
};DisplayRecord retains id, removes the original value: number requirement, and adds value: string. Because the old declaration is no longer part of the intersection, the two types do not compete for the same property.
Choose a union when the value has alternatives
If a value can have one shape or another, use a union (|) rather than an intersection (&). A literal discriminant such as kind lets TypeScript narrow the type based on which alternative is present; the Handbook’s narrowing guide explains this pattern.
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type Circle = { kind: "circle"; radius: number };
type Square = { kind: "square"; sideLength: number };
type Shape = Circle | Square;
function area(shape: Shape): number {
if (shape.kind === "circle") {
return Math.PI * shape.radius ** 2;
}
return shape.sideLength ** 2;
}Here, a shape is a circle or a square, not both simultaneously. Checking kind narrows the union so each branch can use its corresponding property.
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Interfaces are useful when declarations are intended to extend one shared contract and a conflicting property should be caught as an error. The Handbook distinguishes this from intersections: incompatible same-name properties in interface extension produce an error, while intersections combine the property requirements and may produce never. Use extension when that early conflict check better expresses your intent; use Omit when the change is deliberate.
How TypeScript version affects the diagnostic
TypeScript 3.9 introduced stricter checking for intersections and documented that an impossible intersection with conflicting discriminant properties can reduce to never as a whole. As a result, some compiler diagnostics may say that a property does not exist on never, rather than simply displaying that individual property as never. The TypeScript 3.9 release notes describe the change.
Quick Recap
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Pick the type construction that matches the intent
| What you mean | Use | Why |
|---|---|---|
| A value must meet every requirement | Intersection (A & B) |
All constituent constraints apply. |
| One property is intentionally redefined | Omit<Base, "key"> & { key: NewType } |
Removes the old key before adding its replacement. |
| A value may have one of several shapes | Union (A | B), often with a discriminant |
Models alternatives and supports narrowing. |
| Declarations extend one contract and incompatibilities should fail | Interface extension | Reports incompatible extended properties as errors. |
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