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Yes—a library can type-check templates without compiling them as part of a build. In an October 6, 2026 article, Jonas Gauffin describes a separate @relax.js/core check command that uses TypeScript’s compiler API to inspect supported template literals and report type errors before rendering. Its coverage depends on how the template is written and whether the checker can resolve its types.
Why check templates that TypeScript sees as strings?
When a template is embedded in a string or tagged template literal, ordinary TypeScript checking may not validate the expressions inside it. A misspelled view-model property or an invalid event attribute can therefore escape checks that would catch comparable mistakes in TypeScript code.
Gauffin’s described checker reads the project’s tsconfig.json, finds literal compileTemplate calls and html tagged template literals, then resolves supported expressions against TypeScript types. It emits diagnostics in a tsc-like format, with locations pointing to the relevant expression. The described implementation adds a check command rather than a build step; it is not an editor service.
What the checker validates
Template expressions and loops
- Interpolation paths,
ifandunlessexpressions, and handler arguments are checked segment by segment against the relevant type. - Null and undefined values are traversed because the runtime renders them as empty. Array access such as
[0]resolves to the array’s element type; traversal stops atany. - A loop such as
loop="row in rows"requiresrowsto be an array and checksrowas the element type within the loop.
Function calls, events, and pipes
- Calls in interpolations and click handlers are checked against the functions context’s call signature and argument types. The handler’s
eventis typed asEvent. - For an
r-<event>attribute, the checker looks for the correspondingon<event>member on the element’s DOM type, using the same test as the runtime. - Pipes must be built in unless a registry is passed to the call. An interpolation opened with
{{must close on the same line.
Type information for each template form
For compileTemplate, the described API accepts type parameters for the view model passed to rendering and for the functions context. Without type arguments, the checker still performs syntax, event, and pipe checks, but it cannot resolve template paths against those types.
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For html templates, the checker can infer a context type from a bind call in the same file. A bind function returned from, or called in, another file may not have an inferable context type. Gauffin’s examples show the checker catching a misspelled property, an unknown event attribute, and a dotted path where the html tag expects a flat property name.
Which templates are skipped or only partly checked?
The checker needs literal template text at the call site to inspect the expressions described above. Gauffin says it skips templates held in variables, built with ${} substitutions, or loaded from files. A template using compileTemplate without type arguments can still receive the non-path checks described earlier, but not path resolution.
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The checker reports skipped templates and their reasons. A run with skipped templates is not equivalent to a fully checked run: review that summary rather than treating an absence of diagnostics as proof that every template was validated. If dynamic templates are intentional, a project can allow skips; if all templates are expected to be static, asserting that the skipped list is empty makes the limitation visible in routine validation.
How closely does checking match rendering?
The runtime and checker share the expression grammar and event test, but handle HTML structure differently. The runtime uses DOMParser; the checker uses a small scanner under Node without a DOM. The scanner does not enforce browser content-model rules. As a result, malformed markup—for example, a paragraph implicitly closed by a block element or a table row placed outside a table—can make loop-alias scope differ from rendered behavior. Keep templates well formed, especially where loop scope depends on surrounding markup.
How to make the check part of a project’s tests
- Run
@relax.js/core checkagainst the project configuration. The command readstsconfig.jsonand reports diagnostics for supported templates. - In the test suite, call
checkProject(tsconfig)and assert that both the formatted diagnostics and skipped-template list are empty if the project expects every template to be statically checkable. - If dynamic or file-loaded templates are legitimate, decide explicitly how to handle skips rather than silently treating them as checked. The strict no-skips assertion can be omitted where those cases are expected.
Gauffin describes TypeScript as an optional peer dependency loaded for the check command rather than the runtime. That is a claim about the implementation in his October 6, 2026 article, not independent confirmation of the package’s current release behavior.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How this differs from integrated template checking
| Aspect | Framework-integrated checking | Relax.js approach described by Gauffin |
|---|---|---|
| How checking runs | Part of the framework’s compiler | Separate @relax.js/core check command |
| Editor feedback | Angular is described as offering editor integration | No editor squiggles are provided |
| Template coverage | Not specified for Angular in the cited article | Typed literal templates are checked; file-loaded and dynamic templates are not inspected |
| HTML parsing | Not specified for Angular in the cited article | Uses a Node-side scanner that does not apply browser content-model rules |
The practical distinction is not simply whether templates can be checked. It is whether checks run as part of an integrated compiler and editor workflow, or as a separate validation step with explicit limits on which template forms it can analyze.
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