Calling external APIs from Edge Functions in Supabase - Time & Space Complexity

Time Complexity: Calling external APIs from Edge Functions

O(n)

Understanding Time Complexity

When Edge Functions call external APIs, the time taken depends on how many calls are made.

We want to know how the number of API calls grows as input size grows.

Scenario Under Consideration

Analyze the time complexity of this Edge Function calling an external API multiple times.

import { serve } from 'std/server'

serve(async (req) => {
  const data = await req.json()
  const results = []
  for (const item of data.items) {
    const response = await fetch(`https://api.example.com/data/${item.id}`)
    const json = await response.json()
    results.push(json)
  }
  return new Response(JSON.stringify(results))
})

This function loops over input items and calls an external API for each one, collecting results.

Identify Repeating Operations

Look at what repeats as input grows:

• Primary operation: One external API call per input item.
• How many times: Exactly once for each item in the input list.

How Execution Grows With Input

Each new item adds one more API call, so the total calls grow directly with input size.

Input Size (n)	Approx. API Calls/Operations
10	10 calls
100	100 calls
1000	1000 calls

Pattern observation: The number of calls grows in a straight line as input grows.

Final Time Complexity

Time Complexity: O(n)

This means the time grows directly in proportion to the number of input items.

Common Mistake

[X] Wrong: "Calling multiple APIs inside a loop is always constant time because calls happen fast."

[OK] Correct: Each API call takes time and adds up, so more calls mean more total time.

Interview Connect

Understanding how external API calls scale helps you design efficient cloud functions and shows you think about real-world delays.

Self-Check

What if we changed the code to call the external API only once with all item IDs at once? How would the time complexity change?

Practice

1. What is the main purpose of using fetch in Supabase Edge Functions when calling external APIs?

easy

A. To create user interface components

B. To store data permanently in Supabase database

C. To run SQL queries inside the Edge Function

D. To send requests and receive data from external services

Solution

1. Step 1: Understand the role of fetch in Edge Functions

   fetch is used to send HTTP requests to external APIs and get responses back.

2. Step 2: Identify what fetch does not do

   It does not store data permanently or create UI components; it only communicates with external services.

3. Final Answer:

   To send requests and receive data from external services -> Option D

4. Quick Check:

   fetch calls external APIs = C [OK]

Hint: Remember: fetch is for talking to outside services [OK]

Common Mistakes:

• Thinking fetch stores data permanently
• Confusing fetch with database queries
• Assuming fetch creates UI elements

2. Which of the following is the correct syntax to call an external API using fetch inside a Supabase Edge Function?

easy

A. const response = fetch('https://api.example.com/data').then(res => res.json())

B. const response = fetch.get('https://api.example.com/data')

C. const response = fetch('https://api.example.com/data')

D. const response = fetch('https://api.example.com/data').json()

Solution

1. Step 1: Recall the correct usage of fetch

   fetch returns a promise; to get JSON data, you call then(res => res.json()).

2. Step 2: Check each option for syntax correctness

   const response = fetch('https://api.example.com/data') misses handling the promise; B uses invalid method; D incorrectly calls json() directly on fetch.

3. Final Answer:

   const response = fetch('https://api.example.com/data').then(res => res.json()) -> Option A

4. Quick Check:

   fetch + then(res.json()) = A [OK]

Hint: Always handle fetch with then() or await for JSON [OK]

Common Mistakes:

• Calling json() directly on fetch without then or await
• Using fetch.get which does not exist
• Ignoring the promise returned by fetch

3. Consider this Edge Function code snippet:

export async function handler() {
  const response = await fetch('https://api.example.com/data');
  const data = await response.json();
  return new Response(JSON.stringify({ result: data.value }), { status: 200 });
}

If the external API returns { "value": 42 }, what will the Edge Function return?

medium

A. An error because data.value is undefined

B. A response with body '{"result":42}' and status 200

C. A response with body '42' and status 404

D. A response with body '{"value":42}' and status 200

Solution

1. Step 1: Understand the API response and parsing

   The API returns JSON {"value": 42}, which is parsed into data object with data.value = 42.

2. Step 2: Check the returned response from the Edge Function

   The function returns JSON string with key "result" and value data.value (42), status 200.

3. Final Answer:

   A response with body '{"result":42}' and status 200 -> Option B

4. Quick Check:

   API value 42 mapped to result key = D [OK]

Hint: Map API data correctly before returning JSON [OK]

Common Mistakes:

• Returning original API JSON without mapping
• Confusing status codes
• Assuming data.value is undefined

4. You wrote this Edge Function code to call an external API:

export async function handler() {
  const response = fetch('https://api.example.com/data');
  const data = await response.json();
  return new Response(JSON.stringify(data), { status: 200 });
}

What is the main error in this code?

medium

A. Missing await before fetch causing response to be a Promise

B. Using await on response.json() is incorrect

C. Response constructor cannot take JSON string

D. The function should not return a Response object

Solution

1. Step 1: Identify how fetch is called

   fetch returns a Promise, so it must be awaited to get the response object.

2. Step 2: Check usage of await on response.json()

   Awaiting response.json() is correct; the error is missing await on fetch.

3. Final Answer:

   Missing await before fetch causing response to be a Promise -> Option A

4. Quick Check:

   Missing await before fetch causing response to be a Promise = B [OK]

Hint: Always await fetch before calling response methods [OK]

Common Mistakes:

• Not awaiting fetch before response.json()
• Thinking response.json() does not need await
• Misunderstanding Response constructor usage

5. You want to call two different external APIs sequentially inside a Supabase Edge Function and combine their results before returning. Which code snippet correctly does this?

hard

A. const [res1, res2] = await Promise.all([
fetch('https://api1.com/data'),
fetch('https://api2.com/data')
]);
const data1 = await res1.json();
const data2 = await res2.json();
return new Response(JSON.stringify({ first: data1, second: data2 }), { status: 200 });

B. const res1 = fetch('https://api1.com/data');
const data1 = await res1.json();
const res2 = fetch('https://api2.com/data');
const data2 = await res2.json();
return new Response(JSON.stringify({ first: data1, second: data2 }), { status: 200 });

C. const res1 = await fetch('https://api1.com/data');
const data1 = await res1.json();
const res2 = await fetch('https://api2.com/data');
const data2 = await res2.json();
return new Response(JSON.stringify({ first: data1, second: data2 }), { status: 200 });

D. const data1 = fetch('https://api1.com/data').json();
const data2 = fetch('https://api2.com/data').json();
return new Response(JSON.stringify({ first: data1, second: data2 }), { status: 200 });

Solution

1. Step 1: Understand sequential API calls

   To call APIs one after another, await each fetch and parse before next call.

2. Step 2: Check each option for correct async handling

   const res1 = await fetch('https://api1.com/data'); const data1 = await res1.json(); const res2 = await fetch('https://api2.com/data'); const data2 = await res2.json(); return new Response(JSON.stringify({ first: data1, second: data2 }), { status: 200 }); correctly awaits each fetch and json parsing sequentially. const res1 = fetch('https://api1.com/data'); const data1 = await res1.json(); const res2 = fetch('https://api2.com/data'); const data2 = await res2.json(); return new Response(JSON.stringify({ first: data1, second: data2 }), { status: 200 }); misses await on fetch. const [res1, res2] = await Promise.all([ fetch('https://api1.com/data'), fetch('https://api2.com/data') ]); const data1 = await res1.json(); const data2 = await res2.json(); return new Response(JSON.stringify({ first: data1, second: data2 }), { status: 200 }); uses parallel calls, not sequential. const data1 = fetch('https://api1.com/data').json(); const data2 = fetch('https://api2.com/data').json(); return new Response(JSON.stringify({ first: data1, second: data2 }), { status: 200 }); incorrectly calls json() directly.

3. Final Answer:

   const res1 = await fetch('https://api1.com/data');\nconst data1 = await res1.json();\nconst res2 = await fetch('https://api2.com/data');\nconst data2 = await res2.json();\nreturn new Response(JSON.stringify({ first: data1, second: data2 }), { status: 200 }); -> Option C

4. Quick Check:

   Sequential await fetch + json = A [OK]

Hint: Await each fetch and json call in order for sequential calls [OK]

Common Mistakes:

• Not awaiting fetch before json()
• Calling json() directly on fetch
• Mixing parallel and sequential calls incorrectly