javascript - Default values for Vue component props & how to check if a user did not set the prop? - Stack Overflow

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1. How can I set the default value for a component prop in Vue 2? For example, there is a simple movies component that can be used in this way:

<movies year="2016"><movies>



Vueponent('movies', {
    props: ['year'],

    template: '#movies-template',
    ...
}

But, if a user doesn't specify the year:

<movies></movies>

then the component will take some default value for the year prop.

2. Also, what is the best way to check if a user did not set the prop? Is this a good way:

if (this.year != null) {
    // do something
}

or maybe this:

if (!this.year) {
    // do something
}

?

1. How can I set the default value for a component prop in Vue 2? For example, there is a simple movies component that can be used in this way:

<movies year="2016"><movies>



Vue.component('movies', {
    props: ['year'],

    template: '#movies-template',
    ...
}

But, if a user doesn't specify the year:

<movies></movies>

then the component will take some default value for the year prop.

2. Also, what is the best way to check if a user did not set the prop? Is this a good way:

if (this.year != null) {
    // do something
}

or maybe this:

if (!this.year) {
    // do something
}

?

• javascript
• vue.js
• vue-component

Share Improve this question Follow edited Jan 14, 2019 at 14:08 PeraMika asked Nov 1, 2016 at 17:52 PeraMikaPeraMika 3,6881111 gold badges4141 silver badges7070 bronze badges

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4 Answers 4

Sorted by: Reset to default 342

Vue allows for you to specify a default prop value and type directly, by making props an object (see: https://vuejs.org/guide/components.html#Prop-Validation):

props: {
  year: {
    default: 2016,
    type: Number
  }
}

If the wrong type is passed then it throws an error and logs it in the console, here's the fiddle:

https://jsfiddle.net/cexbqe2q/

This is an old question, but regarding the second part of the question - how can you check if the user set/didn't set a prop?

Inspecting this within the component, we have this.$options.propsData. If the prop is present here, the user has explicitly set it; default values aren't shown.

This is useful in cases where you can't really compare your value to its default, e.g. if the prop is a function.

Also something important to add here, in order to set default values for arrays and objects we must use the default function for props:

propE: {
      type: Object,
      // Object or array defaults must be returned from
      // a factory function
      default: function () {
        return { message: 'hello' }
      }
    },

Worth mentioning, that you also can add a custom validator function to every prop like this, to ensure the passed value matches your components specific requirements:

props: {
  year: {
    type: Number,
    required: true
    validator (value) {
      return value > 2015
    }
  }
}

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