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Template Literal Type - Does not evaluate interpolated template strings against type definition #41732
Description
TypeScript Version: 4.1.2
Search Terms:
template literal type template string interpolation number pixel value
Code
const pixelValue: number = 22;
type PixelValueType = `${number}px`;
const pixelString: PixelValueType = `22px`;
// Error here
const pixelStringWithTemplate: PixelValueType = `${pixelValue}px`;Expected behavior:
A template string should be evaluated to determine if it matches a template literal type.
Actual behavior:
The template string is evaluated to type string even though it can match the Template Literal Type
Type 'string' is not assignable to type '`${number}px`'.(2322)
Related Issues:
Activity
jcalz commentedon Dec 1, 2020
Related issues: #40707, #10676
From #40707, if you use a
constassertion the compiler will give a template literal type to a template literal expression, resulting in your desired behavior:Playground link
It looks like your desired behavior is for the compiler to always synthesize a template literal type for a template literal expression the way #10676 does for other literal types, so that
as constis not necessary.a1245582339 commentedon Dec 3, 2020
Why the
${pixelValue}pxcannot be inferred to PixelValueType automatically? It seems nothing problem.RyanCavanaugh commentedon Dec 3, 2020
Anders Hejlsberg (@ahejlsberg) Seems like we should be providing a literal context here?
ahejlsberg commentedon Dec 7, 2020
This is a duplicate of #41631.
ahejlsberg commentedon Dec 9, 2020
Now implemented in #41891.