Import Capabilities

js/realm/capabilities.md

Every import a child realm makes is evaluated against an ordered rule list. Rules come from two sources: the parent's own rules (which the child inherits), and child-specific overrides supplied at construction time. The child's overrides are appended after the parent's, so they can narrow permissions but cannot escalate past what the parent already allows.

Building an import map

ImportMap.deny() starts from a block-all baseline and opens specific exceptions:

import { Realm, ImportMap } from 'fino:realm';

const realm = new Realm({
  entry: './worker.ts',
  overrides: ImportMap.deny([
    { pattern: 'fino:file/path', directive: 'inherit' },
    { pattern: 'fino:context',   directive: 'inherit' },
  ]),
});

This child can import fino:file/path and fino:context. Every other specifier is blocked, including all other fino:* modules.

ImportMap.inherit() starts from an inherit-all baseline and blocks specifics:

const realm = new Realm({
  entry: './worker.ts',
  overrides: ImportMap.inherit([
    { pattern: 'fino:process', directive: 'block' },
  ]),
});

This child inherits everything the parent can import, except fino:process.

Use deny when you want a tight allowlist — the child gets only what you explicitly permit. Use inherit when the child is trusted and you only need to remove a few capabilities.

Rule evaluation — last match wins

Rules are evaluated in order and the last matching rule determines the outcome. Put your broad baseline first and specific overrides after it:

ImportMap.inherit([
  { pattern: 'fino:*',   directive: 'block' },    // block all fino: modules...
  { pattern: 'fino:ffi', directive: 'inherit' },  // ...except ffi (last match wins)
])

If you reverse the order, the broad block would apply last and win over the specific inherit.

ImportMap.deny(overrides) is equivalent to new ImportMap([{ pattern: '*', directive: 'block' }, ...overrides]). ImportMap.inherit(overrides) is equivalent to new ImportMap([{ pattern: '*', directive: 'inherit' }, ...overrides]). You can construct an ImportMap directly when you need precise control over the rule list.

Directives

The directive field in each rule is one of:

• 'inherit' — fall back to the parent's rule for this specifier. The effective behavior depends on whatever the parent allows.
• 'block' — reject the import with a module-not-found error.
• { type: 'remap', target: string } — resolve the import as if the child had imported target instead. Useful for aliasing a virtual specifier to a real module.
• { type: 'source', code: string, source_map: string } — inject an in-memory module with the given source text as the result of the import.
• A Facade instance — expose a parent-backed virtual module. The runtime normalizes the facade to its wire directive automatically.

Per-importer targeting with from

Each rule has an optional from field. When present, the rule only applies when the module initiating the import matches the from pattern:

// Block fino:fs only when imported from a specific module
{ from: 'internal:plugin-host', pattern: 'fino:fs', directive: 'block' }

Without from, the rule applies regardless of which module is doing the importing. The from field accepts the same pattern syntax as pattern, including * and prefix patterns.

Capability narrowing

The runtime enforces that children cannot escalate past what the parent allows. If the parent has blocked a specifier, the child's override list cannot grant access to it — construction throws:

// Throws at construction if internal:* is blocked in the parent
new Realm({
  overrides: ImportMap.deny([
    { pattern: 'internal:*', directive: { type: 'remap', target: 'internal:realm-native' } },
  ]),
  entry: './worker.ts',
});

This is enforced by the Rust-side loader before the child realm is created. There is no way for a JS-level rule to bypass it.

Import inheritance

Child realms inherit the parent's import rules by default. You do not need to re-declare filesystem or network facades in every child: the parent's rules flow down unless the child's import map narrows or replaces them.