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Template Literal Type - Does not evaluate interpolated template strings against type definition #41732
Description
Activity
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:const pixelValue: number = 22; type PixelValueType = `${number}px`; const pixelStringWithTemplate: PixelValueType = `${pixelValue}px` as const; // okay
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.Reacted by William Cooter, Daniel Kezerashvili and Cesare PuliattiRelated 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:const pixelValue: number = 22; type PixelValueType = `${number}px`; const pixelStringWithTemplate: PixelValueType = `${pixelValue}px` as const; // okay
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.Why the
${pixelValue}pxcannot be inferred to PixelValueType automatically? It seems nothing problem.Anders Hejlsberg (@ahejlsberg) Seems like we should be providing a literal context here?
This is a duplicate of #41631.
Now implemented in #41891.
Reacted by Daniel Kezerashvili- locked as resolved and limited conversation to collaborators
on Oct 21, 2025
TypeScript Version: 4.1.2
Search Terms:
template literal type template string interpolation number pixel value
Code
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
stringeven though it can match the Template Literal TypePlayground Link:
https://www.typescriptlang.org/play?#code/MYewdgzgLgBADgSwB4FMA2A1AhmgrigLhjFwFsAjFAJxgF4YAmBgbgChWoBPOFGABWTpseFABVuvegAMAJAG8SFagF84SKW1ahIsRKjQBlKFQRgA5kQH7h+cTzowpTNRq3ho8QYeOmzAdQQoAAtRFFI4NCwoQn4vGzEJB1k5PSEcfFV1NiA
Related Issues: