TypeScript类型交叉类型产生never类型的原因及特定交叉表达式结果的技术咨询

Let's break down why these two intersection type expressions produce their respective results—it all boils down to how TypeScript merges conflicting properties in intersection types.

1. Why { d: true; e: number; } & { d: false; e: number; } resolves to never #

An intersection type (&) requires a value to satisfy all the types it combines. Let's zero in on the d property here:

• The first type enforces d must be true
• The second type enforces d must be false

There’s no possible value that can be both true and false at the same time. Since the d property alone has no valid possible values, the entire combined type has zero valid values too. That’s exactly what the never type represents: a type with no possible runtime values.

Which is why in your code example, you can only assign an error-throwing expression to a variable of type DataTypeNever—there’s no legal value that fits the type:

type DataTypeNever = { d: true; e: number; } & { d: false; e: number; }
let data: DataTypeNever = (() => { throw new Error })() // Only works because this never returns a valid value

2. Why { d: false; e: number; } & { d: false; e: string; } resolves to { d: false, e: never } #

Here, the d property is consistent across both types—both specify d must be false. So TypeScript merges these into a single d: false property with no conflict.

Now look at the e property:

• The first type requires e to be a number
• The second type requires e to be a string

Just like with the d property in the first example, there’s no value that is both a number and a string. So the e property’s type becomes never (no valid values possible).

The combined type ends up as { d: false, e: never } because:

• d has a valid, consistent value (false)
• e has no valid values (never)

Again, this is why you can only assign an error-throwing expression to the e property—there’s no valid number/string hybrid value to use:

type DataType = { d: false; e: number; } & { d: false; e: string; }
let data: DataType = { d: false, e: (() => { throw new Error })() }

内容的提问来源于stack exchange，提问作者Jason