Ns Trace Debugging

Trace: turning on the framework's own diagnostics

@nativescript/core logs rich internals — binding failures, layout passes, navigation, native lifecycle — but only when you enable BOTH the switch and the categories:

import { Trace } from '@nativescript/core';

Trace.enable();                       // step 1 — alone this prints NOTHING
Trace.addCategories(
  Trace.categories.concat(            // step 2 — pick categories
    Trace.categories.Binding,
    'BindingError',                   // see the All caveat below
    Trace.categories.Layout,
    Trace.categories.Navigation,
  ),
);

The full category list: Accessibility, VisualTreeEvents, Layout, Style, ViewHierarchy, NativeLifecycle, Debug, Navigation, Test, Binding, BindingError, Error, Animation, Transition, Livesync, ModuleNameResolver, MediaQuery.

The All that isn't all

Trace.categories.All omits Accessibility and BindingError. The two categories you most want when hunting a silent binding failure or an a11y issue need explicit opt-in — passing All and concluding "no binding errors were logged" is a false negative. Trace.categories.Accessibility is also missing from the shipped TypeScript typings — pass the string 'Accessibility'.

Reading the output

A ConsoleWriter is registered by default; each line is `${category}: ${message}` in the ns run console:

Binding: Binding target: Label(34) targetProperty: text
BindingError: Property 'titel' not found on ViewModel

Grep ns run output for the category prefix. Calling Trace.addWriter(...) adds a writer — output duplicates unless you Trace.clearWriters() first when installing a custom one.

Trace.error() throws by default

The default error handler rethrows — Trace.error(new Error('x')) crashes unless you install a handler first. The handler interface method is spelled handlerError (sic), not handleError:

Trace.setErrorHandler({
  handlerError(err) {           // note the spelling
    console.log('[trace-error]', err?.message ?? err);
  },
});
Trace.error('string input is wrapped into an Error for you');

Also: error-level writes (Trace.write(msg, category, Trace.messageType.error)) bypass the enabled/category filters — they always reach writers.

Practical bootstraps

Debug binding issues: enable + Binding + 'BindingError' (pairs with ns-binding-expressions). Debug "why did my view not re-layout": Layout + ViewHierarchy. Debug navigation event order: Navigation (pairs with ns-page-navigation-lifecycle). Gate it out of release builds with if (__DEV__).

Verified 2026-08 against @nativescript/core 9.1.0-rc.3: category list, the All omissions, default ConsoleWriter format, handlerError spelling and the rethrow default read from packages/core/trace/index.ts (and the typings gap from trace/index.d.ts); not re-run standalone.