javascript - In Vue.js, why isn't my computed property working - Stack Overflow

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In the following code:

new Vue({
  el: '#app',
  puted: {
    myMessage: function() {
      return "hello";
    }
  },
  data: {
    message: this.myMessage
  },
  mounted: function() {
    console.log(this.myMessage);
  }
})

<script src=""></script>

<div id="app">
  <p>{{ message }}</p>
</div>

In the following code:

new Vue({
  el: '#app',
  puted: {
    myMessage: function() {
      return "hello";
    }
  },
  data: {
    message: this.myMessage
  },
  mounted: function() {
    console.log(this.myMessage);
  }
})

<script src="https://unpkg./vue"></script>

<div id="app">
  <p>{{ message }}</p>
</div>

https://jsfiddle/hk49eL38/

if I replace:

this.myMessage with a string (eg "Hello world"), it renders correctly.

But when I use this.myMessage, no text is rendered.

Why is this?

• javascript
• vue.js

Share Improve this question Follow edited Apr 27, 2019 at 12:18 Daniel Beck 21.5k55 gold badges4343 silver badges6060 bronze badges asked Apr 27, 2019 at 3:42 MaryMary 1,15544 gold badges2222 silver badges4646 bronze badges 1

• Think of it like this: data is where you store you initial values, puted is like the name says, results of some putation. – Rudy Commented Apr 27, 2019 at 9:55

Add a ment  | 

4 Answers 4

Sorted by: Reset to default 8

The problem is that you are trying to use the puted property as the initial value of the message data property.

When the data function is called internally by Vue, the puted properties haven't been evaluated yet, this is done very early, before the created hook.

If you try to log the value of the puted property in the beforeCreated hook (the first life-cycle hook) instead of the mounted as in your example, you will see that it is undefined.

See the ponent life-cycle:

(*) Diagram cropped for brevity

Computed properties are evaluated in the "Init injections & reactivity" step in the above diagram.

Computed property values cannot be used as the initial value of a property of data, and normally they depend on data property values and other external and reactive values (e.g. Vuex getters, Vue router parameters, etc).

You shouldn't use this before writing myMessage, as VueJS doesn't work like that. It already considers everything in this context. You should do it like this:

<script src="https://unpkg./vue"></script>

<div id="app">
  <p>{{ myMessage }}</p>
</div>

Try setting the value of myMesssage to message when ponent is mounted.

mounted: function() {
    this.message = this.myMessage
}

Try to change the data message in the puted my message

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