Environment variables ($env) in Svelte - Performance & Optimization

Performance: Environment variables ($env)

MEDIUM IMPACT

This affects page load speed and bundle size by controlling which environment variables are included in the client bundle.

Using environment variables in a Svelte app

Svelte

import { PUBLIC_API_URL } from '$env/static/public';

console.log(PUBLIC_API_URL);

Imports only the needed variable statically, allowing bundler to tree-shake unused vars and reduce bundle size.

📈 Performance GainSaves 10-50kb in bundle, improves LCP by reducing JS parsing and execution time.

Using environment variables in a Svelte app

Svelte

import { env } from '$env/dynamic/public';

console.log(env);

Imports all public environment variables dynamically, increasing bundle size and delaying initial render.

📉 Performance CostAdds 10-50kb to bundle depending on env size, blocks rendering longer on initial load.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Dynamic import of all env vars	Minimal	0	Low but delayed due to JS parsing	[X] Bad
Static import of specific env vars	Minimal	0	Low and fast	[OK] Good

Rendering Pipeline

Environment variables imported statically are replaced at build time, reducing runtime overhead. Dynamic imports include all variables, increasing JS parsing and delaying style and layout calculations.

→Parsing

→Script Evaluation

→Style Calculation

→Layout

⚠️ BottleneckScript Evaluation due to larger JS bundle from dynamic env imports

Core Web Vital Affected

LCP

This affects page load speed and bundle size by controlling which environment variables are included in the client bundle.

Optimization Tips

1Import only the environment variables you need using static imports.

2Avoid dynamic imports of all environment variables to keep bundle size small.

3Check your JS bundle size and script evaluation time to ensure fast page load.

Performance Quiz - 3 Questions

What is the main performance drawback of importing all environment variables dynamically in Svelte?

A It reduces CSS selector efficiency.

B It causes more DOM reflows.

C It increases bundle size and delays initial rendering.

D It blocks network requests.

DevTools: Network and Performance

How to check: Open DevTools, go to Network tab, reload page and check JS bundle size. Then use Performance tab to record page load and check scripting time.

What to look for: Look for large JS bundles and long scripting times indicating heavy dynamic env imports. Smaller bundles and faster scripting indicate good static env usage.

Practice

1. What is the main reason to use environment variables in a Svelte project?

easy

A. To avoid using any external libraries

B. To write all code inside a single file

C. To make the app run faster by skipping variables

D. To store settings outside the code for safety and flexibility

Solution

1. Step 1: Understand environment variables purpose

   Environment variables keep sensitive or changeable settings outside the main code, so you can change them without editing code.

2. Step 2: Connect to safety and flexibility

   This separation helps keep secrets safe and lets you adjust settings easily for different environments like development or production.

3. Final Answer:

   To store settings outside the code for safety and flexibility -> Option D

4. Quick Check:

   Environment variables = safety and flexibility [OK]

Hint: Environment variables keep secrets outside code [OK]

Common Mistakes:

• Thinking environment variables speed up the app
• Believing environment variables are for code organization only
• Confusing environment variables with libraries

2. Which import statement correctly accesses public environment variables in a Svelte component?

easy

A. import { PUBLIC_API_KEY } from '$env/static/public';

B. import { API_KEY } from '$env/static/private';

C. import { API_KEY } from '$env/dynamic/private';

D. import { PUBLIC_API_KEY } from '$env/dynamic/public';

Solution

1. Step 1: Identify public environment variable import

   Public variables are accessed from '$env/static/public' when known at build time.

2. Step 2: Match variable prefix and import path

   Public variables must start with 'PUBLIC_' and be imported from '$env/static/public' for static access.

3. Final Answer:

   import { PUBLIC_API_KEY } from '$env/static/public'; -> Option A

4. Quick Check:

   Public vars use static/public import [OK]

Hint: Public vars use $env/static/public import [OK]

Common Mistakes:

• Using private import paths for public variables
• Forgetting the PUBLIC_ prefix
• Mixing dynamic and static imports incorrectly

3. Given this Svelte code snippet, what will be displayed if the environment variable PUBLIC_APP_NAME is set to 'MyApp'?

import { PUBLIC_APP_NAME } from '$env/static/public';

<h1>Welcome to {PUBLIC_APP_NAME}!</h1>

medium

A. Welcome to MyApp!

B. Welcome to PUBLIC_APP_NAME!

C. Welcome to !

D. Error: Cannot find variable PUBLIC_APP_NAME

Solution

1. Step 1: Understand import of PUBLIC_APP_NAME

   The variable PUBLIC_APP_NAME is imported from '$env/static/public', so its value is available at build time.

2. Step 2: Check usage in template

   The variable is used inside the <h1> tag, so it will display the actual value 'MyApp'.

3. Final Answer:

   <h1>Welcome to MyApp!</h1> -> Option A

4. Quick Check:

   Static public env var shows actual value [OK]

Hint: Static public vars show real value in template [OK]

Common Mistakes:

• Expecting variable name instead of value
• Assuming variable is undefined without import
• Confusing static and dynamic imports

4. You try to access a private environment variable in a Svelte component using this code:

import { SECRET_KEY } from '$env/static/private';

console.log(SECRET_KEY);

But you get an error during build. What is the likely cause?

medium

A. SECRET_KEY must start with PUBLIC_ prefix

B. You must import from '$env/dynamic/public' instead

C. Private variables cannot be imported in client-side code

D. You forgot to declare SECRET_KEY in .env file

Solution

1. Step 1: Understand private env variable usage

   Private environment variables are only available on the server side and cannot be imported into client-side code.

2. Step 2: Recognize build error cause

   Importing private variables in a component that runs in the browser causes build errors because secrets must stay server-only.

3. Final Answer:

   Private variables cannot be imported in client-side code -> Option C

4. Quick Check:

   Private vars are server-only [OK]

Hint: Private vars only on server, not client [OK]

Common Mistakes:

• Adding PUBLIC_ prefix to private vars
• Using dynamic public import for private vars
• Assuming missing .env declaration causes build error

5. You want to show a greeting message in your Svelte app that changes based on a public environment variable PUBLIC_GREETING. You also want to update this message without rebuilding the app. Which approach should you use?

hard

A. Import PUBLIC_GREETING from $env/static/public and use it directly

B. Import PUBLIC_GREETING from $env/dynamic/public and use it in your component

C. Store PUBLIC_GREETING in a local variable inside the component

D. Hardcode the greeting message in the component

Solution

1. Step 1: Understand static vs dynamic public env variables

   Static public variables are fixed at build time, so changes require rebuilding the app.

2. Step 2: Use dynamic public env for runtime updates

   Dynamic public variables from '$env/dynamic/public' reflect environment changes without rebuilding, ideal for updating messages.

3. Final Answer:

   Import PUBLIC_GREETING from $env/dynamic/public and use it in your component -> Option B

4. Quick Check:

   Dynamic public env allows runtime updates [OK]

Hint: Use $env/dynamic/public for runtime env changes [OK]

Common Mistakes:

• Using static public import for dynamic needs
• Storing env vars in local variables without import
• Hardcoding messages instead of using env vars