Use immediate Option Instead of Duplicate Initial Call

Impact: LOW - Calling a function manually at setup and also in a watcher leads to duplicate code. The immediate: true option runs the watcher callback immediately with the current value, combining both behaviors.

Task Checklist

Use { immediate: true } instead of calling handler in onMounted/created
Consider watchEffect as an alternative (always runs immediately)
Remember immediate callback receives undefined as oldValue on first run

Incorrect:

<script setup>
import { ref, watch, onMounted } from 'vue';

const userId = ref(1);
const userData = ref(null);

// BAD: Duplicate calls - once in onMounted, once in watch
async function loadUser(id) {
  const response = await fetch(`/api/users/${id}`);
  userData.value = await response.json();
}

onMounted(() => {
  loadUser(userId.value); // Initial call
});

watch(userId, (newId) => {
  loadUser(newId); // On change
});
</script>

// Options API - BAD: Duplicate in created and watch
export default {
  data() {
    return { userId: 1, userData: null };
  },
  created() {
    this.loadUser(); // Initial call
  },
  watch: {
    userId() {
      this.loadUser(); // On change - duplicate!
    },
  },
  methods: {
    async loadUser() {
      const response = await fetch(`/api/users/${this.userId}`);
      this.userData = await response.json();
    },
  },
};

Correct:

<script setup>
import { ref, watch } from 'vue';

const userId = ref(1);
const userData = ref(null);

// GOOD: Single watch with immediate runs on mount and on change
watch(
  userId,
  async (newId) => {
    const response = await fetch(`/api/users/${newId}`);
    userData.value = await response.json();
  },
  { immediate: true },
);
</script>

// Options API - GOOD: Using immediate option
export default {
  data() {
    return { userId: 1, userData: null };
  },
  watch: {
    userId: {
      async handler(newId) {
        const response = await fetch(`/api/users/${newId}`);
        this.userData = await response.json();
      },
      immediate: true,
    },
  },
};

Alternative: watchEffect

<script setup>
import { ref, watchEffect } from 'vue';

const userId = ref(1);
const userData = ref(null);

// watchEffect always runs immediately - no option needed
watchEffect(async () => {
  const response = await fetch(`/api/users/${userId.value}`);
  userData.value = await response.json();
});
</script>

Handling oldValue on Initial Run

<script setup>
import { ref, watch } from 'vue';

const count = ref(0);

watch(
  count,
  (newVal, oldVal) => {
    // On first run with immediate: true
    // oldVal is undefined
    if (oldVal === undefined) {
      console.log('Initial run, count is:', newVal);
    } else {
      console.log(`Count changed from ${oldVal} to ${newVal}`);
    }
  },
  { immediate: true },
);
</script>

One-Time Watch with once Option

<script setup>
import { ref, watch } from 'vue';

const data = ref(null);

// Run only once when data becomes available
watch(
  data,
  (value) => {
    if (value) {
      initializeWithData(value);
      // Watcher automatically stops after this
    }
  },
  { once: true },
);
</script>

Reference

Vue.js Watchers - Eager Watchers

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