Fix: Prev Next Link incorrectly linking to "Nightly Builds" page by d…

[anantoghosh]

Issue

Currently, on many handbook pages, the "Previous" and "Next" link blocks at the bottom of the page incorrectly point to the "Nightly Builds" page by default when there should be no link at all.

For example:

• https://www.typescriptlang.org/docs/handbook/intro.html
• https://www.typescriptlang.org/docs/handbook/2/modules.html
• https://www.typescriptlang.org/docs/handbook/asp-net-core.html
• https://www.typescriptlang.org/docs/handbook/nightly-builds.html
• ... and others.

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Cause

This issue occurs due to the context variables previousID and nextID being passed as undefined when no relevant links are found.
These variables are used in the eq filter here:

TypeScript-Website/packages/typescriptlang-org/src/templates/documentation.tsx

Line 269 in ac68b8b

prev: file(id: { eq: $previousID } ) {

The context variables are set here:

TypeScript-Website/packages/typescriptlang-org/lib/bootup/ingestion/createPagesForDocumentation.ts

Lines 120 to 121 in ac68b8b

	previousID,
	nextID,

Gatsby's graphql query explicitly only handles string, number, boolean and null values in eq. When undefined is passed, the filter gets ignored, which in this case results in the "Nightly Builds" page being linked by default.

GraphQL explorer output:

• When undefined is passed
  

• When null is passed
  

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Solution

This PR resolves the issue by setting the previousID and nextID variables to null by default instead of undefined. If the next or previous page is found, these variables will be updated accordingly; otherwise, they remain null.

Fixed handbook/intro.html: