JavaScript Code Style

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ScandiPWA follows a strict JavaScript style guide for maintainability and consistency
Code style recommendations in ScandiPWA consist of two main categories: functional programming, which is enforced to make the codebase easier to maintain, and ScandiPWA best practices, which have been implemented to guarantee that code is extensible, both by overriding the theme and writing plugins.
We strongly recommend you use ESlint to check your code style. This article was written to help you understand the code style rules we enforce and write better code.
Writing Maintainable Code
Keep Functions Short
Functions should do only one thing, and do it well. If you notice that a function has become longer than necessary, consider breaking it up into parts. Not only will this make your codebase easier to navigate and manage, but it will also make your theme easier to extend via plugins and theme overrides.
This is especially relevant when writing functions that return JSX. Breaking them down into multiple functions can reduce nesting, improve readability, and make them easier to extend.
Avoid writing long functions such as this:
component/MyAccountOverlay/MyAccountOverlay.component.js (renderCreateAccount function)
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    renderCreateAccount() {
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        const {
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            state,
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            onCreateAccountAttempt,
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            onCreateAccountSuccess,
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            handleSignIn
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        } = this.props;
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​
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        return (
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            <>
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                <Form
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                  key="create-account"
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                  onSubmit={ onCreateAccountAttempt }
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                  onSubmitSuccess={ onCreateAccountSuccess }
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                  onSubmitError={ onCreateAccountAttempt }
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                >
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                    <fieldset block="MyAccountOverlay" elem="Legend">
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                        <legend>{ __('Personal Information') }</legend>
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                        <Field
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                          type="text"
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                          label={ __('First Name') }
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                          id="firstname"
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                          name="firstname"
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                          autocomplete="given-name"
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                          validation={ ['notEmpty'] }
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                        />
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                        <Field
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                          type="text"
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                          label={ __('Last Name') }
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                          id="lastname"
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                          name="lastname"
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                          autocomplete="family-name"
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                          validation={ ['notEmpty'] }
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                        />
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                        <Field
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                          type="checkbox"
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                          value="is_subscribed"
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                          label={ __('Subscribe to newsletter') }
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                          id="is_subscribed"
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                          mix={ { block: 'MyAccountOverlay', elem: 'Checkbox' } }
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                          name="is_subscribed"
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                        />
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                    </fieldset>
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                    <fieldset block="MyAccountOverlay" elem="Legend">
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                        <legend>{ __('Sign-Up Information') }</legend>
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                        <Field
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                          type="text"
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                          label={ __('Email') }
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                          id="email"
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                          name="email"
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                          autocomplete="email"
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                          validation={ ['notEmpty', 'email'] }
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                        />
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                        <Field
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                          type="password"
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                          label={ __('Password') }
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                          id="password"
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                          name="password"
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                          autocomplete="new-password"
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                          validation={ ['notEmpty', 'password'] }
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                        />
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                        <Field
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                          type="password"
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                          label={ __('Confirm password') }
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                          id="confirm_password"
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                          name="confirm_password"
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                          autocomplete="new-password"
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                          validation={ ['notEmpty', 'password', 'password_match'] }
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                        />
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                    </fieldset>
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                    <div block="MyAccountOverlay" elem="Buttons">
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                        <button
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                          block="Button"
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                          type="submit"
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                        >
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                            { __('Sign up') }
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                        </button>
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                    </div>
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                </Form>
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                <article block="MyAccountOverlay" elem="Additional" mods={ { state } }>
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                    <section>
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                        <h4>{ __('Already have an account?') }</h4>
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                        <button
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                          block="Button"
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                          mods={ { likeLink: true } }
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                          onClick={ handleSignIn }
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                        >
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                            { __('Sign in here') }
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                        </button>
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                    </section>
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                </article>
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            </>
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        );
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    }
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Issues:
Code is harder to understand and navigate
Re-ordering or re-using sub-components requires a lot of changes
Instead, try breaking your code into smaller functions:
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    // define helper functions here...
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    renderCreateAccount() {
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        const {
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            state,
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            onCreateAccountAttempt,
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            onCreateAccountSuccess
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        } = this.props;
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​
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        return (
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            <>
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                <Form
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                  key="create-account"
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                  onSubmit={ onCreateAccountAttempt }
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                  onSubmitSuccess={ onCreateAccountSuccess }
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                  onSubmitError={ onCreateAccountAttempt }
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                >
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                    { this.renderPersonalInformationFieldset() }
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                    { this.renderCredentialFieldset() }
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                    <div block="MyAccountOverlay" elem="Buttons">
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                        <button
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                          block="Button"
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                          type="submit"
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                        >
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                            { __('Sign up') }
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                        </button>
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                    </div>
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                </Form>
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                <article block="MyAccountOverlay" elem="Additional" mods={ { state } }>
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                    { this.renderSignInLink() }
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                </article>
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            </>
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        );
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    }
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Advantages:
Each function has a clear responsibility and is easy to change
The structure of the resulting JSX is more clear
The component is more extensible via plugins and overrides
Separate Concerns
Containers should be responsible for business logic, and components should be responsible for presentation logic. Making this distinction will make it easier to structure your code.
Use Destructuring
​Destructuring enables you to "unpack" certain values from an object such as the state or props.
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const { product: { name, sku } = {} } = this.props;
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// you can now use name instead of this.props.product.name
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// and sku instead of this.props.product.sku
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​
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// if this.props.product is undefined, it will get the default value {}.
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// name and sku will be undefined, which you can easily check...
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// but at least the page won't crash for accessing the property of an
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// undefined value
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ScandiPWA prefers destructuring all required variables at the beginning of a function over direct field access, as it offers several benefits:
More concise code with reduced repetition if the same value is used multiple times
Ability to provide default values
By moving destructuring to the first line of each function, the dependencies of that function are clear
Use Meaningful Names
To make code easier to understand, avoid generic names such as x or abbreviations such as prdct. Give meaningful names that describe what the variable/function/class is for.
Avoid Magic Numbers
It can be tempting to pass a hard-coded literal value to a function:
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CSS.setVariable(
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    this.draggableRef,
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    'animation-speed',
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    `${ Math.abs(distance * 300) }ms`
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);
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However, the purpose of the number 300 is not clear, and might confuse a developer looking at this for the first time.
Instead, consider creating a constant to describe the meaning of the value:
component/Slider/Slider.component.js (simplified excerpt)
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export const ANIMATION_DURATION = 300;
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​
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CSS.setVariable(
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    this.draggableRef,
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    'animation-speed',
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    `${ Math.abs(distance * ANIMATION_DURATION) }ms`
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);
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Advantages:
The intention of the code is clear, and the math makes sense
ANIMATION_DURATION can be easily reused if needed
The duration can be adjusted without worrying about breaking something
Functional Programming
Functional programming aims to make code easier to reason about and maintain by avoiding mutable values. It can make your codebase easier to navigate as well as more concise and elegant.
Avoid let; Use const
Use const for every variable and do not reassign values.
Avoid let and reassigments:
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let price = 4.5;
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price = '#x27; + price;
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price = `This product costs ${ price }.`
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Potential problems that can occur as the codebase grows and price is used more times:
price can have multiple meanings; a developer looking at line 1 might miss the reassignment and assume price is a number
The type of price can change and can get hard to guess
Any code needing the original value of price can't get it
Instead prefer:
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const price = 4.5;
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const formattedPrice = '#x27; + price;
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const message = `This product costs ${ formattedPrice }.`
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Benefits:
You are forced to give a meaningful name to each variable, making your intentions more clear
The values are immutable and easier to reason about
Any previous value can be re-used if necessary 
Avoid Loops
In functional programming, loops are discouraged in favor of iterative functions that signal intent better. In addition, they are often elegant and concise.
If you need to iterate over the array to produce a single value, such as the sum, maximum value, or even an object containing some of the array's values, you can use reduce.
Avoid using a loop to combine the array's elements:
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const items = [2, 4, 24, 42];
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let sum = 0;
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for (let i = 0; i < items.length; i++) {
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    sum += items[i];
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}
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Instead prefer reduce:
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const items = [2, 4, 24, 42];
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const sum = items.reduce((sum, item) => sum + item);
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Advantages:
More concise and elegant
Avoids an imperative loop and a mutable value
If you see this for the first time, it might seem counter-intuitive. Perhaps MDN's docs will help!
If you need to transform the values of the array into different values, use map
Avoid using a loop to transform an array:
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const items = [2, 4, 24, 42];
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const newItems = []
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for (let i = 0; i < items.length; i++) {
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    newItems.push(`Item ID: ${ items[i] }`);
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}
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Instead, use map:
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const items = [2, 4, 24, 42];
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const newItems = items.map(item => `Item ID: ${ item }`);
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Advantages:
More concise and elegant
Intentions are clear - this is a typical use of map
In general, you should be able to write code in JavaScript without needing to use loops at all. Following this practice will result in cleaner and more maintainable code.
Working with Arrays
For code to be easier to reason about, you should avoid mutating arrays. Instead, it is preferred to create new arrays with the values you want. This will often be more concise, and make the data flow easier to follow. In addition, it is consistent with how React and Redux handles state - instead of giving you access to modify the state directly, you are expected to provide new values for the state.
​MDN Web Docs are a great reference resource for JavaScript. Here you can find a summary of how array functions can be used to write functional-programming-style code, with links to the MDN documentation.
Creating a new array by transforming each item
​map: calls the function for each value and returns the array of results
map is often used in JSX when you need to render an array of items. You can "map" or transform the array into an array of react elements by calling map with a function that accepts each item and returns JSX. Example:
component/ProductCustomizableOptions/ProductCustomizableOptions.component.js (excerpt)
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        return options.map((option, key) => (
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            <ProductCustomizableOption
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              option={ option }
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              key={ key }
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            />
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        ));
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Reducing the array to 1 value
​reduce: combines all elements into 1 new value, according to the specified reducing function
​some: returns true if and only if the specified function returns true for at least 1 element in the array
​every: returns true if and only if the specified function returns true for all elements
Copying part of the array
​filter: returns a copy of the array with only those items that meet the specified condition
​slice: returns a portion of the array specified by indices
Finding an item in the array
​find: returns the first element that meets the specified condition
​includes: returns true if and only if the specified element is in the array
Reordering the array
​sort: sorts the array according to the specified ordering function
​reverse: reverses the items of the array
While sort and reverse return the re-ordered array, they also change the original array. For this reason, they are not ideal from the functional programming perspective.
Most of the time, mutating the original array might be acceptable. If you do not wish to mutate the array, create a copy of it and re-order the copy instead.
Adding an item to the array
Instead of mutating the array, you can create a new array with the additional value:
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const newItem = 42;
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const array = [1, 2, 3];
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const newArray = [newItem, ...array];
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Removing an item from an array
Instead of mutating the array, you can create a new array with the value removed, using filter:
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const itemToRemove = 42;
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const array = [42, 1, 2, 3];
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const newArray = array.filter(item => item !== itemToRemove);
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// can also filter by index (the second parameter in the filter function)
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Extensibility Best Practices
In addition to using functional programming, ScandiPWA also recommends that you follow certain guidelines to ensure that your code can be easily extended by plugins and theme overrides.
Export Everything
If a plugin or theme override were to extend your code, they would need to have access to your classes and functions. A theme override might want to base its code on your class without needing to copy-paste it. For this reason, it is strongly recommended that you export all top-level classes, functions and values that you define.
Add Namespaces
Plugins can only affect values that have namespaces. For this reason, it is highly recommended that you add a namespace to all functions and classes:
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/** @namespace Route/Checkout/Container/mapStateToProps */
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export const mapStateToProps = (state) => ({
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    totals: state.CartReducer.cartTotals,
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    customer: state.MyAccountReducer.customer
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});
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​
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/** @namespace Route/Checkout/Container/mapDispatchToProps */
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export const mapDispatchToProps = (dispatch) => ({
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    updateMeta: (meta) => dispatch(updateMeta(meta)),
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    resetCart: () => CartDispatcher.then(
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        ({ default: dispatcher }) => dispatcher.updateInitialCartData(dispatch)
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    ),
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});
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​
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/** @namespace Route/Checkout/Container */
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export class CheckoutContainer extends PureComponent {
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    // [...]
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​
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    saveGuestEmail() {
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        const { email } = this.state;
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        const { updateEmail } = this.props;
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        const guestCartId = BrowserDatabase.getItem(GUEST_QUOTE_ID);
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        const mutation = CheckoutQuery.getSaveGuestEmailMutation(email, guestCartId);
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​
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        updateEmail(email);
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        // even dynamically created functions should have namespaces
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        return fetchMutation(mutation).then(
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            /** @namespace Route/Checkout/Container/saveGuestEmailFetchMutationThen */
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            ({ setGuestEmailOnCart: data }) => data,
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            this._handleError
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        );
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    }
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}
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​
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export default connect(mapStateToProps, mapDispatchToProps)(CheckoutContainer);
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​
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Namespaces should consist of:
The alias of the directory (Component/Store/Route, etc)
The name of this component (Checkout in this case)
The current file's responsibility, if applicable (Component/Container for components, Dispatcher/Reducer for stores)
The name of the target function/class, or some other meaningful name if it is anonymous
ScandiPWA Conventions
Follow the File Structure
ScandiPWA has a specific file structure: the source directory contains 7 sub-directories, such as component, route, store, etc. To keep code organized, it is advised to avoid deviating from this file structure. This means that you are allowed to create new components, routes, etc, but you should not add any new "main" directories.
One Class Per File
Each file should define at most one class. Adding additional classes can make the codebase harder to navigate.
​