Issues with utilizing a generic type in an Arrow function in the Typescript Playground

Question

Issues with utilizing a generic type in an Arrow function in the Typescript Playground

When I try to use a generic type with an arrow function in Typescript Playground, I get an error message saying Cannot find name 'T'

For more details, check out this link

function hasAllProperties <T>(obj: any, props: (keyof T)[]): obj is T {
    return props.every((prop) => obj.hasOwnProperty(prop))
}

// This code throws an error and won't compile 
const hasAllPropertiesArrow = <T>(obj: any, props: (keyof T)[]): obj is T => {
    return props.every((prop) => obj.hasOwnProperty(prop))
}

Since I am new to generic types, I believe the issue lies in my lack of understanding rather than a bug in the TypeScript playground. Could someone please help me rewrite the normal function as an arrow function?

typescript typescript-generics

Answer 1

Answer №1

The TypeScript parser is limited in its design, as detailed in microsoft/TypeScript#15713. The usage of const x = <T>() can mislead the compiler into interpreting <T> as a JSX tag, leading to error messages like

JSX element 'T' has no corresponding closing tag.

To resolve this issue, if JSX/TSX support is not required, one can eliminate the --jsx compiler option by referring to this Playground link.

For those requiring JSX support, a workaround involves adding a trailing comma after introducing the T type parameter:

const hasAllPropertiesArrow = <T,>(obj: any, props: (keyof T)[]): obj is T => {
    return props.every((prop) => obj.hasOwnProperty(prop))
}

This comma serves no functional purpose but aids in preventing parser confusion.

Explore the code using this Playground link.

Answer 2

The TypeScript parser is limited in its design, as detailed in microsoft/TypeScript#15713. The usage of const x = <T>() can mislead the compiler into interpreting <T> as a JSX tag, leading to error messages like

JSX element 'T' has no corresponding closing tag.

To resolve this issue, if JSX/TSX support is not required, one can eliminate the --jsx compiler option by referring to this Playground link.

For those requiring JSX support, a workaround involves adding a trailing comma after introducing the T type parameter:

const hasAllPropertiesArrow = <T,>(obj: any, props: (keyof T)[]): obj is T => {
    return props.every((prop) => obj.hasOwnProperty(prop))
}

This comma serves no functional purpose but aids in preventing parser confusion.

Explore the code using this Playground link.

Issues with utilizing a generic type in an Arrow function in the Typescript Playground

Answer №1

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