How to use computed props in Vue.js with TypeScript?

Question

There is a lot of documentation how to interact with Vue.js using the JavaScript language and just a little about TypeScript. The question is how do you define computed props in a vue component if it is written in TypeScript?

According the official example, computed is an object with functions which will be cached based on their dependent props.

Here is an example I made:

import Vue from 'vue';
import { Component } from "vue-property-decorator";

@Component({})
export default class ComputedDemo extends Vue {
    private firstName: string = 'John';
    private lastName: string = 'Doe';
    private computed: object = {
        fullName(): string {
            return `${this.firstName} ${this.lastName}`;
        },
    }
}

And html:

<div>
    <h1>Computed props ts demo</h1>
    <ul>
        <li>First name: {{firstName}}</li>
        <li>Last name: {{lastName}}</li>
        <li>Together: {{fullName}}</li>
    </ul>
</div>

The third list item outputs nothing. Can anybody tell me how to define computed in this case, please?

question from:https://stackoverflow.com/questions/51982139/how-to-use-computed-props-in-vue-js-with-typescript

Answer 1

You can use property accessors to declare computed properties. See Vue Class Component. The getter will be triggered as soon as you type in the input.

For example:

<template>
    <div>
        <input type="text" name="Test Value" id="" v-model="text">

        <label>{{label}}</label>
    </div>

</template>

<script lang="ts">
import { Component, Vue, Watch } from "vue-property-decorator";

@Component({})
export default class About extends Vue {
    private text = "test";

    get label() {
        return this.text;
    }
}
</script>

---

Update for Vue Composition Api

<template>
  <div>
    <input type="text" name="Test Value" id v-model="text" />

    <label>{{label}}</label>
  </div>
</template>

<script lang="ts">
import { defineComponent, ref, computed } from "@vue/composition-api";

export default defineComponent({
  setup() {
    const text = ref("test");

    const label = computed(() => {
      return text.value;
    });

    return {
      text,
      label
    };
  }
});
</script>