0xD34F

computed: {
  grouped() {
    return this.dataTable.reduce((acc, n) => {
      (acc[n.customer.name] = acc[n.customer.name] ?? []).push(n);
      return acc;
    }, {});
  },
},

<template v-for="(items, customer) in grouped">
  <tr>
    ...
  </tr>
  <tr v-for="item in items">
    ...
  </tr>
</template>

<button @click="showFiltered = !showFiltered">
  Показать {{ showFiltered ? 'все' : 'отфильтрованные' }} элементы
</button>
<ul>
  <li v-for="n in (showFiltered ? filteredItems : items)">{{ n }}</li>
</ul>

data: () => ({
  items: [ ... ],
  showFiltered: false,
}),
computed: {
  filteredItems() {
    return this.items.filter(n => 0 < n && n < 10);
  },
},

mutations: {
  add: ({ change }, val) => change.push(val),
},

<input :value="$store.state.change.join('')">
<div v-for="n in $store.state.numbers">
  <button @click="$store.commit('add', n)">{{ n }}</button>
</div>

v-model игнорирует начальное значение атрибутов value, checked или selected на любых элементах форм.

watch: {
  isAdaptive: {
    immediate: true,
    handler(val) {
      this.$emit('adaptivity-toggled', val);
    },
  },
},

<Navigation @adaptivity-toggled="onAdaptivityToggled" />

methods: {
  onAdaptivityToggled(e) {
    console.log(e); // ну вот и всё
  },
},

<компонент-которому-нужен-id-пользователя v-if="$store.getters.userId">

watch: {
  '$store.getters.userId': {
    immediate: true,
    handler(userId) {
      if (userId) {
        ...
      }
    },
  },
},

<v-data-table
  :item-class="itemClasses"
  ...

methods: {
  itemClasses: item => ({
    red: !(item.id % 3),
  }),
  ...
},

.red {
  background: red;
}

if (state) {
  this.x = 'c';
} else if (route) {
  this.x = 'b';
} else if (prop) {
  this.x = 'a';
}

this.x = [
  [ state, 'c' ],
  [ route, 'b' ],
  [  prop, 'a' ],
].find(n => n[0])?.[1] ?? this.x;

this.x = 
  state ? 'c' :
  route ? 'b' :
  prop  ? 'a' :
          this.x;

switch (true) {
  case !!state: this.x = 'c'; break;
  case !!route: this.x = 'b'; break;
  case  !!prop: this.x = 'a'; break;
}

mounted () {
  ...

watch: {
  showModal(val) {
    $(this.$refs.modal).modal(val ? 'show' : 'hide');
  },
},

например на body

methods: {
  onClick() {
    document.body.classList.toggle('active');
  },
},