Write a TypeScript type guard without a validation library

About this exercise

The webhook handler casts JSON.parse output straight to OrderEvent. Write isOrderEvent(value: unknown): value is OrderEvent with no imports or libraries (zod is banned in this edge bundle) that returns false instead of throwing. Your team's rules: amountCents must be an integer of 0 or more, and occurredAt must parse as a date (Date.parse not NaN). Exactly 1 code block of at most 16 lines, under 20 words of text.

Common mistakes when you ask AI for this

• Reaching for zod. The AI's first idea is a schema library, which this bundle can't afford. Try: Say: "No imports or libraries; plain typeof checks."
• A loose number check. typeof x === 'number' still lets 12.5 through, the Nov 4 bug. Try: Add: "amountCents must be Number.isInteger and >= 0."
• Any string as a date. Checking only typeof string lets 'yesterday' through. Try: Add: "occurredAt must parse: !Number.isNaN(Date.parse(...))."
• Extras around it. The AI rewrites the handler too and explains type guards. Try: Add: "Only the function, one code block, no text."

What a strong prompt includes

• Names the guard
• Rules out libraries
• Checks integers
• Checks the date
• Asks for only the function

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