Defaults And Fallbacks

A default block supplies fields for one named type or enum variant.
A container fallback belongs to one array or map declaration and changes missing-index results from optional to required.
Default blocks and collection fallbacks solve different problems and use different syntax.

Defaults For Named Types

code

1type ExpenseDraft {2  title: string3  amount: number4  note: string?5}67default ExpenseDraft {8  title: "Untitled expense"9  amount: 010}

A default target must exist, each field must exist on that target, and every default value must match its field type. Only one default block may target the same named type.

Defaults For Enum Variants

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1enum ExpenseStatus {2  Draft,3  Open,4  Submitted,5  Approved { approved_at: time },6  Rejected { reason: string },7  Settled8}910default ExpenseStatus.Rejected {11  reason: "No reason supplied"12}

Target the structured variant with Enum.Variant. A unit variant has no fields to default.

Container Fallbacks

code

1type BalanceLabel {2  label: string3  amount: number4}56BalanceLabel { label: "Unknown", amount: 0 } [] rows = []7BalanceLabel first = rows[0]89BalanceLabel { label: "Unknown", amount: 0 } {} by_member = {}10BalanceLabel missing = by_member["missing"]

The fallback value appears before the collection marker. Missing array positions and map keys return that value, so the result is not optional.

Fallback Ownership Rules

The fallback must satisfy the named element type.
It is evaluated once when the declaration runs.
It belongs only to that declaration.
Assigning the collection to a normal array or map does not copy fallback behavior.
Normal collections without a fallback keep optional index results.
An empty object fallback must be spaced: BalanceLabel {} []. Compact BalanceLabel{}[] means an array of maps.

Choose The Right Mechanism

Need	Use
Fill omitted fields on a named value	`default Type { ... }`
Fill fields on a structured enum variant	`default Enum.Variant { ... }`
Replace one absent scalar expression	`optional ?? fallback`
Handle present or absent branches	`with` or `without`
Replace a missing array index or map key	A declaration-level container fallback.
Represent no elements	An empty array or map.

Common Mistakes

Mistake	Correction
Use a default when absence is meaningful	Keep the value optional.
Assume a fallback persists data	Use an entity default and explicit save when persistence is intended.
Use container fallbacks in Page Script	Initialize browser state explicitly.
Define two defaults for the same target	Keep one unambiguous default block.

Continue Learning

Optionals And Absence Use ? and ?? for scalar absence.Enums Define the variants targeted by enum defaults.