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Rustam Apay
Rustam Apay

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5. Modularity 2. Passing props. Computed template variables

#webdev #javascript #beginners #tutorial

Components (real)

For now our components are colored rectangles with static content (text). In real components content is dynamic, passed by props.

Props are external parameters (variables) that we pass from parent to child.

The idea of a component is that we receive external data from parent (props), and put them into html-like template with empty slots for these props inside {{ }}.

Passing prop (v-bind)

Open index.html, copy a code of one of the keys, that we commented before, and paste it to the Key component template

Key.js

const Key = {
    template: `
    <div class="key">
        <div class="main">1</div>
        <div class="shifted">!</div>
    </div>`
}

export default Key
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We should tell Key about a new prop. And use the prop in the template.

Key.js

const Key = {
    template: `
    <div class="key">
        <div class="main">
            {{ keyContent.main }}
        </div>
        <div class="shifted">
            {{ keyContent.shifted }}
        </div>
    </div>`,
    props: {
        keyContent: Object
    }
}

export default Key
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  1. We told component about a prop props: {keyContent: Object}
  2. We told template where to place props by {{ }}.

Next we will pass prop from a parent.

Open keyboardData/en.js and copy data for a single key:

keyboardData/en.js

{
    code: 'Digit1',
    main: '1',
    shifted: '!',
    mainName: 'one',
    shiftedName: 'exclamation mark'
}
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We will pass this object to Key component as a prop keyContent from the parent Keyboard.

Keyboard.js

import Key from './Key.js'

const keyData =
    /*paste here copied before data*/
    {
        code: 'Digit1',
        main: '1',
        shifted: '!',
        mainName: 'one',
        shiftedName: 'exclamation mark'
    }

const Keyboard = {
    template: `
    <div class="keyboard">
        Keyboard
        <vue-key :keyContent="keyData" />
    </div>`,
    components: {
        'vue-key': Key
    },
    /* put external data inside component */
    data() {
        return { keyData }
    }
}

export default Keyboard
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Here we:

  • pasted key data to const keyData
  • made the component see this data in method data(){}
  • passed keyData as a prop to child Key component <vue-key :keyContent="keyData" />.

Notice, when we pass prop, we use colon : before its name.

<vue-key :keyContent="keyData" />
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In that case framework interprets “keyData” as a variable name.

Otherwise, (without :) it would be interpreted as a string:

<vue-key keyContent="keyData" /> -- Key component will receive string “keyData” instead of the object keyData.

Result:

Image description

Return to Keyboard.js. We already have all keys data in keyboardData/en.js so let’s import and use it, instead of a single key data:

Keyboard.js

import Key from './Key.js'
import keyboardData from '../keyboardData/en.js'

const Keyboard = {
    template: `
    <div class="keyboard">
        Keyboard
        <vue-key :keyContent="keyboardData[1][0]" />
        <vue-key :keyContent="keyboardData[1][1]" />
        <vue-key :keyContent="keyboardData[1][2]" />
        <vue-key :keyContent="keyboardData[1][3]" />
        <vue-key :keyContent="keyboardData[1][4]" />
        <vue-key :keyContent="keyboardData[1][5]" />
    </div>`,
    components: {
        'vue-key': Key
    },
    data() {
        return { keyboardData }
    }
}

export default Keyboard
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Now we have less code and more keys. If change Key component, all keys in the app will be changed.

Result

Image description

Computed (template variables)

If we try to display first row keyboardData[0] (Esc, F1, F2, …)

    ...
    template: `
    <div class="keyboard">
        <vue-key :keyContent="keyboardData[0][0]" />
        <vue-key :keyContent="keyboardData[0][1]" />
        <vue-key :keyContent="keyboardData[0][2]" />
        <vue-key :keyContent="keyboardData[0][3]" />
        <vue-key :keyContent="keyboardData[0][4]" />
        <vue-key :keyContent="keyboardData[0][5]" />
    </div>`,
    ...
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we will get empty yellow rectangles:

Image description

That’s because these keys doesn’t have main or shifted values:

;[
    { code: 'Escape', label: 'Esc' },
    { code: 'F1' },
    { code: 'F2' },
    { code: 'F3' },
    { code: 'F4' },
    { code: 'F5' }
]
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So we need to compute them from other params: code and label. Vue component has especial property computed for such cases.

Key.js

const Key = {
    ...,
    props: {
        keyContent: Object
    },
    /* add: */
    computed: {
        main() {
            /*
            Code line below is called `destructuring`,
            because we destructure an object `this.keyContent`
            into 3 separate constants.
            */
            const { main, label, code } = this.keyContent

            return label || main || code
/*
|| is a logical `or` operator. The line above is `or chain`. If `label` exists, it will be returned. If label doesn't exist, but `main` exists -- will be returned `main`. If label and main don't exist, will be returned `code`.
*/
        },
        shifted() {
            const { shifted } = this.keyContent
            return shifted
        }
    }
}

export default Key
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We added to component object a new property computed with 2 methods: main() and shifted(). We also changed template to use this new values:

{{keyboardData.main}} —> {{main}}

{{keyboardData.shifted}} —> {{shifted}}

Result:

Image description

Before we output all rows, remove all temporary styles, that we added to see how component hierarchy works. Remove these lines from the end of styles.css.

styles.css

#app {
    background-color: red;
    padding: 10px;
}

.langSwitcher {
    background-color: green;
    padding: 10px;
}

.keyboard {
    background-color: blue;
    padding: 10px;
    display: flex;
}

.key {
    background-color: yellow;
    padding: 10px;
    color: black;
}
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Let’s output all rows from our data model. Put code below to Keyboard template

Keyboard.js template

<div class="keyboard">
    <div class="row row-1">
        <vue-key :keyContent="keyboardData[0][0]" />
        <vue-key :keyContent="keyboardData[0][1]" />
        <vue-key :keyContent="keyboardData[0][2]" />
        <vue-key :keyContent="keyboardData[0][3]" />
        <vue-key :keyContent="keyboardData[0][4]" />
        <vue-key :keyContent="keyboardData[0][5]" />
    </div>
    <div class="row row-2">
        <vue-key :keyContent="keyboardData[1][0]" />
        <vue-key :keyContent="keyboardData[1][1]" />
        <vue-key :keyContent="keyboardData[1][2]" />
        <vue-key :keyContent="keyboardData[1][3]" />
        <vue-key :keyContent="keyboardData[1][4]" />
        <vue-key :keyContent="keyboardData[1][5]" />
    </div>
    <div class="row row-3">
        <vue-key :keyContent="keyboardData[2][0]" />
        <vue-key :keyContent="keyboardData[2][1]" />
        <vue-key :keyContent="keyboardData[2][2]" />
        <vue-key :keyContent="keyboardData[2][3]" />
        <vue-key :keyContent="keyboardData[2][4]" />
    </div>
</div>
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We wrapped rows with <div class="row row-{{index}}">...</div>.

Result:

Image description

Last line looks not correct.

For languages with upper case letters (e.g. Cyrillic, Latin alphabets), we should show in main slot shifted value (uppercase), and don't show main value at all. Otherwise, our keyboard will look unrealistic. Let's add a function getKeyLabels(keyContent) that does all this work for us:

Key.js

const getKeyLabels = keyContent => {
    const { main = '', shifted = '', label, code } = keyContent
    const isUpperCaseLang = main.toUpperCase() === shifted
    const mainOutput = isUpperCaseLang ? shifted : main
    const shiftedOutput = isUpperCaseLang ? '' : shifted

    return {
        main: label || mainOutput || code,
        shifted: shiftedOutput
    }
}

const Key = {
    template: `
    <div class="key">
        <div class="main">{{main}}</div>
        <div class="shifted">{{shifted}}</div>
    </div>`,
    props: {
        keyContent: Object
    },
    computed: {
        main() {
            const { main } = getKeyLabels(this.keyContent)
            return main
        },
        shifted() {
            const { shifted } = getKeyLabels(this.keyContent)
            return shifted
        }
    }
}

export default Key
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In main() and shifted() we use the new function getKeyLabels.

Result is ok:

Image description

Differences between old code and new code

Entire code after the chapter

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