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vue call method in html

<input type="text" class="form-control" v-model="user.userName" :placeholder="t('un')" required>

vue call method within method

To access a method from within a method 
<script>
var app = new Vue({
  // ...
  methods: {
    f1: function() {},
    f2: function() {},
    f3: function() { this.f1() + this.f2(); }
  }
})
</script>