# Passing Data to Components

Vue Test Utils provides several ways to set data and props on a component, to allow you to fully test the component's behavior in different scenarios.

In this section, we explore the data and props mounting options, as well as VueWrapper.setProps() to dynamically update the props a component receives.

# The Password Component

We will demonstrate the above features by building a <Password> component. This component verifies a password means certain criteria, such as length and complexity. We will start with the following and add features, as well as tests to make sure the features are working correctly:

const Password = {
  template: `
    <div>
      <input v-model="password">
    </div>
  `,
  data() {
    return {
      password: ''
    }
  }
}

The first requirement we will add is a minimum length.

# Using props to set a minimum length

We want to reuse this component in all our projects, each of which may have different requirements. For this reason, we will make the minLength a prop which we pass to <Password>:

We will show an error is password is less than minLength. We can do this by creating an error computed property, and conditionally rendering it using v-if:

const Password = {
  template: `
    <div>
      <input v-model="password">
      <div v-if="error">{{ error }}</div>
    </div>
  `,
  props: {
    minLength: {
      type: Number
    }
  },
  computed: {
    error() {
      if (this.password.length < this.minLength) {
        return `Password must be at least ${this.minLength} characters.`
      }
      return
    }
  }
}

To test this, we need to set the minLength, as well as a password that is less than that number. We can do this using the data and props mounting options. Finally, we will assert the correct error message is rendered:

test('renders an error if length is too short', () => {
  const wrapper = mount(Password, {
    props: {
      minLength: 10
    },
    data() {
      return {
        password: 'short'
      }
    }
  })

  expect(wrapper.html()).toContain('Password must be at least 10 characters')
})

Writing a test for a maxLength rule is left as an exercise for the reader! Another way to write this would be using setValue to update the input with a password that is too short. You can learn more in [Forms]./forms).

# Using setProps

Sometimes you may need to write a test for a side effect of a prop changing. This simple <Show> component renders a greeting if the show prop is true.

<template>
  <div v-if="show">{{ greeting }}</div>
</template>

<script>
import axios from 'axios'

export default {
  props: {
    show: {
      type: Boolean,
      default: true
    }
  },
  data() {
    return {
      greeting: 'Hello'
    }
  }
}
</script>

To test this fully, we might want to verify that greeting is rendered by default. We are able to update the show prop using setProps(), which causes greeting to be hidden:

import { mount } from '@vue/test-utils'
import Show from './Show.vue'

test('renders a greeting when show is true', async () => {
  const wrapper = mount(Show)
  expect(wrapper.html()).toContain('Hello')

  await wrapper.setProps({ show: false })

  expect(wrapper.html()).not.toContain('Hello')
})

We also use the await keyword when calling setProps(), to ensure that the DOM has been updated before the assertions run.

# Conclusion

  • use the props and data mounting options to pre-set the state of a component.
  • Use setProps() to update a prop during a test.
  • Use the await keyword before setProps() to ensure the Vue will update the DOM before the test continues.
  • Directly interacting with your component can give you greater coverage. Consider using setValue or trigger in combination with data to ensure everything works correctly.