Add IsUnion<T>

[willium]

Found this super helpful in my project, we should add it :)

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[sindresorhus]

How do you use it? What problems does it solve?

[willium]

export type Field<T extends SemanticType = SemanticType> = {
	readonly id: FieldId
	readonly type: T
	readonly name?: string
	readonly description?: string
	readonly isHidden?: boolean
}

export const isField = <T extends SemanticType = SemanticType>(
	val: unknown,
	type?: true extends IsUnion<T> ? T[] : T,
): val is Field<T> => {
	if (!isObject(val)) return false
	if (!isPlainObject(val)) return false
	const field = val as Field
	if (!field.id || typeof field.id !== "string") return false
	if (!field.type || typeof field.type !== "string") return false
	if (Array.isArray(type) && !type.includes(field.type)) return false
	if (type && field.type !== type) return false
	if (field.name && typeof field.name !== "string") return false
	if (field.description && typeof field.description !== "string") return false
	if (field.isHidden !== undefined && typeof field.isHidden !== "boolean") {
		return false
	}
	return true
}

[papb]

@willium Can you explain?

[willium]

I'm restricting the allowed type of Field by the types of value(s) passed in as a second parameter

[sindresorhus]

Duplicate of #236