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Choose between reactive and template-driven forms
| Area | Reactive forms | Template-driven forms |
|---|---|---|
| Model setup | Defined explicitly in the component class | Built from directives in the template |
| Validation setup | Validator functions are attached to controls or groups | HTML validation attributes are matched to Angular validator directives |
| Data flow | Synchronous | Asynchronous |
| Best fit | Forms that are central, complex, reusable, or heavily tested | Basic forms whose logic can remain in the template |
| Testing | Less setup and less dependence on change detection | More setup and manual change-detection considerations |
These are Angular’s documented distinctions; an existing project’s architecture and conventions also matter. See Angular’s forms overview.
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Add built-in validation
Reactive forms
Define the form model in the component and pass validator functions to a control. Angular’s reactive forms guide documents this model-driven approach. For example, a required field can be configured with Validators.required; combine validators when a field needs more than one rule.
Template-driven forms
Add supported HTML attributes such as required or minlength to the input. Angular matches these attributes to validator directives. Use a template variable referencing NgModel or the form to inspect validity and validation errors. The template-driven forms guide covers this directive-based setup.
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Write and display useful errors
A validator returns either an error object or null. Check both the specific error key and whether the user has interacted with the control before displaying a message. For example, show a required-field message when the control has a required error, and a separate message when it fails a minimum-length rule. Angular’s examples use dirty or touched state so users are not confronted with errors before interacting with a field.
Use text that explains how to fix the input; do not rely on color alone. A form’s validity can also govern whether submission is available, while inline messages identify which fields need attention. The Angular validation guide includes examples of checking control state and error keys.
Validate related fields together
When a rule depends on more than one value—for example, whether two entries agree—use a custom cross-field validator. In reactive forms, attach the validator to the relevant group so it can inspect sibling controls and report an error at the form-group level. In template-driven forms, implement the validator in a directive registered with NG_VALIDATORS. In either style, make the resulting group-level error visible in a place that explains which related values need correction.
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Use asynchronous validation only when needed
An async validator returns a Promise or Observable, often to check a value against a backend. Angular runs asynchronous validators only after synchronous validators pass, which avoids remote checks for values that already fail local rules.
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By default, async validation may make a backend request on each value change. For an expensive check, configure the control’s update timing as updateOn: 'blur' or updateOn: 'submit' when waiting for a blur or form submission better fits the interaction. This reduces repeated checks while changing when validation feedback appears. Consult the validation guide for async validator behavior and update timing.
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