You may find yourself needing to link two fields together; when one is validated as another field's value has changed. One such usage is when you have both a password and confirm_password field, where you want to confirm_password to error out when password's value does not match; regardless of which field triggered the value change.
Imagine the following userflow:
In this example, the form will still have errors present, as the "confirm password" field validation has not been re-ran to mark as accepted.
To solve this, we need to make sure that the "confirm password" validation is re-run when the password field is updated. To do this, you can add a onChangeListenTo property to the confirm_password field.
<script>
import { createForm } from '@tanstack/svelte-form'
const form = createForm(() => ({
defaultValues: {
password: '',
confirm_password: '',
},
// ...
}))
</script>
<div>
<form.Field name="password">
{#snippet children(field)}
<label>
<div>Password</div>
<input
value={field.state.value}
onChange={(e) => field.handleChange(e.target.value)}
/>
</label>
{/snippet}
</form.Field>
<form.Field
name="confirm_password"
validators={{
onChangeListenTo: ['password'],
onChange: ({ value, fieldApi }) => {
if (value !== fieldApi.form.getFieldValue('password')) {
return 'Passwords do not match'
}
return undefined
},
}}
>
{#snippet children(field)}
<div>
<label>
<div>Confirm Password</div>
<input
value={field.state.value}
onchange={(e) => field.handleChange(e.target.value)}
/>
</label>
{#each field.state.meta.errors as err}
<div>{err}</div>
{/each}
</div>
{/snippet}
</form.Field>
</div>
<script>
import { createForm } from '@tanstack/svelte-form'
const form = createForm(() => ({
defaultValues: {
password: '',
confirm_password: '',
},
// ...
}))
</script>
<div>
<form.Field name="password">
{#snippet children(field)}
<label>
<div>Password</div>
<input
value={field.state.value}
onChange={(e) => field.handleChange(e.target.value)}
/>
</label>
{/snippet}
</form.Field>
<form.Field
name="confirm_password"
validators={{
onChangeListenTo: ['password'],
onChange: ({ value, fieldApi }) => {
if (value !== fieldApi.form.getFieldValue('password')) {
return 'Passwords do not match'
}
return undefined
},
}}
>
{#snippet children(field)}
<div>
<label>
<div>Confirm Password</div>
<input
value={field.state.value}
onchange={(e) => field.handleChange(e.target.value)}
/>
</label>
{#each field.state.meta.errors as err}
<div>{err}</div>
{/each}
</div>
{/snippet}
</form.Field>
</div>
This similarly works with onBlurListenTo property, which will re-run the validation when the field is blurred.