Borislav Hadzhiev
Sat Mar 19 2022·2 min read
Photo by Alice Rouse
The error "Conversion of type 'X' to type 'Y' may be a mistake because neither
type sufficiently overlaps with the other" occurs when we use a type assertion
with incompatible types. To solve the error, widen the type to unknown
first,
e.g. myVar as unknown as MyType
.
Here is an example of how the error occurs.
const num = 5; // ⛔️ Error: Conversion of type 'number' to type 'string' // may be a mistake because neither type sufficiently overlaps // with the other. If this was intentional, convert the //expression to 'unknown' first.ts(2352) const str: string = num as string;
The cause of the error is - the num
variable stores a value of type number
,
so we aren't allowed to use a
type assertion
with an incompatible type such as string
.
To solve the error, we have to widen the number's type to unknown
first and
then narrow it down to string
.
const num = 5; const str: string = num as unknown as string;
We widened the type of the num
variable to
unknown.
The unknown
type is the type-safe counterpart of any.
unknown
type.We had to widen the type to unknown
first, because values of type number
and
not assignable to values of type string
and vice versa.
For example, if the TypeScript compiler thinks that the num
variable might be
of type number
or string
, it would allow us to use a type assertion
directly.
// 👇️ const numOrStr: "hello" | 100 const numOrStr = Math.random() > 0.5 ? 'hello' : 100; // ✅ Works fine const str: string = numOrStr as string;
The numOrStr
variable might be a string or a number, but we are still able to
use a type assertion with the string
type because the types string
and
string | number
overlap.
numOrStr
variable fromstring | number
to string
, which is allowed because there is some overlap.Similarly, if you have a more specific value with a literal type, you are able to use a type assertion to widen its type.
// 👇️ const num: 42 const num = 42; // 👇️ const result: number const result = num as number;
The num
variable has a
literal type
of 42
.
number
, because there is overlap between the types (42
is a number).However if there is no overlap between the types, we have to widen the type to
unknown
before we are able to use a type assertion to an incompatible type.
const num = 42; // 👇️ const result: string const result = num as unknown as string;
Type assertions are used when we have information about the type of a value that TypeScript can't know about.
There are many valid use cases for widening a value's type to unknown
, so we
can use a type assertion with an incompatible type, e.g. incorrect typings of
third party modules.