Compatibility baseline

A mini app runs in JavaScriptCore on iOS and V8 on Android. That is not a design choice Sollar made; it is a legal one it inherited.

Apple’s Guideline 2.5.6 requires that apps browsing the web “use the appropriate WebKit framework and WebKit JavaScript.” The Embedded Browser Engine Entitlement that would allow otherwise exists only in the European Union, and qualifying for it requires 90% pass rates on the Web Platform Tests, 80% on Test262, a memory-safe implementation language and a 30-day CVE remediation commitment.

So the engines differ, and they will keep differing. What Sollar can do is name a floor and test against it.

The floor

	Baseline
JavaScript	ECMAScript 2022
CSS	Everything in Baseline “Widely available” as of 2024
iOS	WKWebView on iOS 16.4+
Android	Android System WebView 114+

Anything at or below this floor works on every device Sollar supports. Anything above needs feature detection.

What ES2022 gives you

Class fields and private methods, at(), Object.hasOwn(), top-level await, Error.cause, Array.prototype.findLast(), RegExp match indices. Modules, async/await, optional chaining and nullish coalescing are all far below the floor.

Above the floor — check first

Feature	Note
`Array.prototype.group` / `groupBy`	Shipped at different times on the two engines. Detect.
`Temporal`	Not on the floor. Use a library.
Decorators	Compile them; do not ship them raw.
`structuredClone`	Available, but with divergent handling of some types.
Web Locks	Not on iOS.
`showOpenFilePicker`	Chromium only. Fall back to `<input type="file">`.
`container-queries`	Above the floor on the oldest supported iOS. Detect.
`:has()`	Same.

if ('group' in Array.prototype) { /* … */ } else { /* … */ }
if (CSS.supports('selector(:has(a))')) { /* … */ }

Feature detection, never user-agent sniffing. The user agent inside a mini app is deliberately uninformative, and treating it as a capability signal will produce wrong answers on both platforms.

Known divergences

	JavaScriptCore (iOS)	V8 (Android)
Stack trace format	`Error.stack` differs in shape
`toLocaleString` output	ICU version differs; the string differs
Regex Unicode property escapes	Both support them; edge cases differ
Timer clamping in background	More aggressive on iOS
Number formatting near precision limits	Differs in the last digit

The practical rule: never parse a formatted string you produced with toLocaleString. Format for display, compute on the underlying value. This is the single most common source of a bug that appears on one platform only.

Conformance suite

sollar test --conformance

Runs Sollar’s own suite against whichever runtime is attached — dev server, iOS simulator, Android emulator, or a physical device. It covers the ES2022 floor, the CSS baseline, every sollar.* method, and the permission paths.

The permission tests run identically in all three environments. Development, trial and production share one authorisation code path; what changes between environments is data and endpoints, never the permission check. Environment-divergent permission behaviour is one of the six vulnerability categories this platform is designed against — see Security model.

Version policy

The floor rises at most once a year, announced at least two release cycles ahead.
A raised floor never breaks an installed mini app; it changes what new submissions may assume.
Removing anything from the bridge requires a major version and a deprecation window.

Secure context

Mini apps run at a synthetic origin over https, so they are secure contexts and crypto.subtle, service workers and the rest are available.