<b-form-textarea
v-model="text"
id="mytextarea"
placeholder="Введите"
class="form-control form-control-lg"
rows="10"
>
</b-form-textarea>
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...
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document.getElementById('mytextarea').value.split('\n').length-1
computed: {
linesCount() {
return 1 + (this.text.match(/\n/g)?.length ?? 0);
},
},
<div>{{ linesCount }}</div>