Environment variables on Deploy in Deno - Step-by-Step Execution

Concept Flow - Environment variables on Deploy

Start Deployment

↓

Set Env Variables

↓

Deploy App

↓

App Reads Env Variables

↓

App Uses Env Variables

↓

App Runs with Config

↓

End

This flow shows how environment variables are set before deployment, then read and used by the Deno app during runtime.

Execution Sample

Deno

console.log(Deno.env.get('API_KEY'));
// Output depends on deployed env variable

This code prints the value of the environment variable 'API_KEY' when the app runs.

Execution Table

Step	Action	Env Variable State	Output
1	Start deployment process	No env variables set yet	No output
2	Set API_KEY='12345' in deployment config	API_KEY='12345'	No output
3	Deploy app with env variables	API_KEY='12345'	No output
4	App starts and reads API_KEY	API_KEY='12345'	No output
5	console.log prints API_KEY value	API_KEY='12345'	12345 printed
6	App runs using API_KEY for config	API_KEY='12345'	App uses key internally
7	End of execution	API_KEY='12345'	Execution complete

💡 App finishes running after using environment variables set during deployment

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
API_KEY	undefined	'12345'	'12345'	'12345'

Key Moments - 2 Insights

Why does console.log(Deno.env.get('API_KEY')) print undefined if I run locally without setting env variables?

Because environment variables must be set in your local environment or deployment config before running. See execution_table step 1 vs step 2 where API_KEY is set only at step 2.

Can I change environment variables inside the running Deno app?

No, environment variables are read-only inside the app. They must be set before deployment or app start. The execution_table shows the variable state does not change after step 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of API_KEY at step 3?

A '12345'

B undefined

C null

D empty string

💡 Hint

Check the 'Env Variable State' column at step 3 in the execution_table.

At which step does the app actually print the API_KEY value?

A Step 2

B Step 4

C Step 5

D Step 6

💡 Hint

Look for the 'Output' column mentioning printed value in the execution_table.

If you forget to set API_KEY before deployment, what will console.log(Deno.env.get('API_KEY')) output?

A '12345'

B undefined

C Error thrown

D Empty string

💡 Hint

Refer to variable_tracker start value and key_moments explanation about missing env variables.

Concept Snapshot

Environment variables are set before deploying a Deno app.
Use Deno.env.get('VAR_NAME') to read them inside the app.
They provide configuration without hardcoding secrets.
Must be set in deployment environment or local shell.
Cannot be changed at runtime inside the app.
App reads them once at start and uses for config.

Full Transcript

This visual trace shows how environment variables work in Deno deployment. First, environment variables like API_KEY are set in the deployment configuration before the app is deployed. When the app starts, it reads these variables using Deno.env.get. The example code prints the API_KEY value, which is '12345' as set. The execution table tracks each step from deployment start, setting variables, deploying, app start, reading variables, printing output, and running with config. The variable tracker shows API_KEY is undefined initially and then set to '12345' after deployment config. Key moments clarify common confusions like why variables print undefined locally if not set, and that variables cannot be changed inside the running app. The quiz questions test understanding of when variables are set, when output happens, and what happens if variables are missing. The snapshot summarizes the key points for quick reference.

Practice

1. What is the main purpose of environment variables when deploying a Deno application?

easy

A. To keep secrets and settings outside the code for security and flexibility

B. To write all configuration directly inside the source code

C. To make the application run faster by hardcoding values

D. To store user data permanently on the server

Solution

1. Step 1: Understand environment variables role

   Environment variables store secrets and settings outside the code to avoid exposing sensitive data.

2. Step 2: Recognize deployment best practices

   Keeping secrets outside code allows changing settings without modifying the app, improving security and flexibility.

3. Final Answer:

   To keep secrets and settings outside the code for security and flexibility -> Option A

4. Quick Check:

   Environment variables = external secrets/settings [OK]

Hint: Environment variables hold secrets outside code [OK]

Common Mistakes:

• Thinking environment variables are for storing user data
• Hardcoding secrets inside source code
• Believing environment variables speed up code execution

2. Which of the following is the correct way to read an environment variable named API_KEY in a Deno script?

easy

A. const key = getenv("API_KEY");

B. const key = process.env.API_KEY;

C. const key = Deno.env.get("API_KEY");

D. const key = env.API_KEY;

Solution

1. Step 1: Recall Deno environment variable syntax

   Deno uses Deno.env.get("NAME") to access environment variables.

2. Step 2: Compare options

   const key = Deno.env.get("API_KEY"); matches Deno syntax; others are from Node.js or incorrect.

3. Final Answer:

   const key = Deno.env.get("API_KEY"); -> Option C

4. Quick Check:

   Deno.env.get("API_KEY") = correct syntax [OK]

Hint: Use Deno.env.get("VAR_NAME") to read env variables [OK]

Common Mistakes:

• Using Node.js syntax like process.env in Deno
• Calling undefined functions like getenv
• Accessing env variables without Deno.env.get

3. Given the following Deno code snippet deployed with environment variable PORT=8080, what will be the output?

const port = Deno.env.get("PORT") || "3000";
console.log(`Server running on port ${port}`);

medium

A. Server running on port 3000

B. Server running on port 8080

C. Server running on port undefined

D. Error: PORT is not defined

Solution

1. Step 1: Check environment variable value

   The environment variable PORT is set to "8080" in deployment.

2. Step 2: Evaluate fallback logic

   The code uses Deno.env.get("PORT") || "3000", so it uses "8080" because PORT exists.

3. Final Answer:

   Server running on port 8080 -> Option B

4. Quick Check:

   PORT=8080 overrides default 3000 [OK]

Hint: Env var set? Use it; else fallback value [OK]

Common Mistakes:

• Assuming fallback value always prints
• Expecting error if env var exists
• Confusing undefined with fallback

4. You deployed a Deno app but reading environment variables returns undefined. Which fix will solve this problem?

medium

A. Restart the app without setting variables

B. Change Deno.env.get to process.env

C. Hardcode variables inside the source code

D. Set environment variables in the deployment platform settings

Solution

1. Step 1: Identify cause of undefined env variables

   Undefined means variables are not set in the deployment environment.

2. Step 2: Apply correct fix

   Setting environment variables in deployment platform ensures Deno.env.get can read them.

3. Final Answer:

   Set environment variables in the deployment platform settings -> Option D

4. Quick Check:

   Env vars must be set in deploy settings [OK]

Hint: Always set env vars in deployment platform [OK]

Common Mistakes:

• Using Node.js syntax in Deno
• Hardcoding secrets instead of setting env vars
• Restarting app without setting variables

5. You want to deploy a Deno app that uses an environment variable MODE to switch between 'development' and 'production'. Which approach correctly uses environment variables to change app behavior safely?

hard

A. Set MODE in deployment settings and use Deno.env.get("MODE") in code to conditionally run features

B. Hardcode const MODE = "production" in source code and redeploy to change

C. Use a config file checked into source with MODE value

D. Ask users to input MODE at runtime every time

Solution

1. Step 1: Understand environment variable usage for modes

   Environment variables allow switching modes without changing code.

2. Step 2: Choose safe and flexible method

   Setting MODE in deployment and reading it with Deno.env.get enables safe conditional logic.

3. Final Answer:

   Set MODE in deployment settings and use Deno.env.get("MODE") in code to conditionally run features -> Option A

4. Quick Check:

   Env vars control modes safely without code changes [OK]

Hint: Use env vars to toggle modes, not hardcoding [OK]

Common Mistakes:

• Hardcoding mode and redeploying for changes
• Storing secrets or modes in config files checked into source
• Asking users to input mode every run