Using logical operators with potential undefined values in TypeScript

I've encountered an error that I'm struggling to fully understand. It seems to be related to the fact that a certain value might be undefined, requiring me to check if it exists. However, when setting up the code as shown below, errors occur. Surprisingly, replacing {showDescription && with 

{isFeatureBlock && description &&
inline solves the issue. But why doesn't it work when assigned to a const? Note: This is a simplified version focusing on the main problem at hand.

The Code

isFeatureBlock is a boolean and description is an optional string

const showDescription = isFeatureBlock && description

{showDescription && (
   <BlockDescription>
     {description}
   </BlockDescription>
 )}

The Error

  1. 'Argument of type 'string | undefined' is not assignable to parameter of type 'string'. Type 'undefined' is not assignable to type 'string'
javascripttypescript

Answer №1

There are no issues with this code, however in your actual codebase, it seems likely that the error arises when you rely on description where a valid string is expected. The addition of description && resolves this issue by narrowing its type on the right side of &&.

In the case of description && SOME_EXP, TypeScript effectively refines the type of description from string | undefined to string within SOME_EXP. This inference is possible because TypeScript can determine statically that description will not be undefined within that context. Notably, this only applies when the variable is used directly on the left side of the && expression – using a constant assigned from description will not yield the same result.

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