Reading files with promises (fs.promises) in Node.js - Step-by-Step Execution

Concept Flow - Reading files with promises (fs.promises)

Start

↓

Call fs.promises.readFile

↓

Returns a Promise

↓

Promise Pending

↓

Promise Resolved

Yes→Get file content

↓

Use content

↓

Promise Rejected

No→Handle error

↓

End

This flow shows how reading a file with fs.promises.readFile returns a promise that either resolves with file content or rejects with an error.

Execution Sample

Node.js

import { promises as fs } from 'fs';

async function readFile() {
  const data = await fs.readFile('example.txt', 'utf8');
  console.log(data);
}

readFile();

This code reads 'example.txt' asynchronously using promises and prints its content.

Execution Table

Step	Action	Promise State	Result	Next Step
1	Call fs.readFile('example.txt', 'utf8')	Pending	No data yet	Wait for file read
2	File read completes successfully	Resolved	File content string	Assign to 'data'
3	Assign file content to 'data'	Resolved	data contains file text	Execute console.log(data)
4	Print file content to console	Resolved	File content shown in console	Function ends
5	Function readFile completes	Resolved	No errors	Program ends

💡 Promise resolves when file is read successfully, ending the async function.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
data	undefined	file content string	file content string	file content string

Key Moments - 2 Insights

Why does the code use 'await' before fs.readFile?

Because fs.readFile returns a promise, 'await' pauses execution until the promise resolves, so 'data' gets the file content instead of a promise object. See execution_table step 2 and 3.

What happens if the file does not exist?

The promise rejects with an error, which would throw unless caught. This is not shown in the sample but would happen after step 1 if the file is missing.

Visual Quiz - 3 Questions

Look at the execution table, what is the state of the promise at step 1?

A Rejected

B Pending

C Resolved

D Undefined

💡 Hint

Check the 'Promise State' column at step 1 in the execution_table.

At which step is the file content assigned to the variable 'data'?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Action' and 'Result' columns in execution_table for when 'data' changes.

If the file read failed, which step would NOT happen?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

If the promise rejects, the console.log in step 4 would not run.

Concept Snapshot

Use fs.promises.readFile to read files asynchronously.
It returns a promise that resolves with file content.
Use 'await' inside async functions to get the content.
Handle errors with try/catch or .catch() on the promise.
Example: const data = await fs.readFile('file.txt', 'utf8');

Full Transcript

This lesson shows how to read files in Node.js using fs.promises.readFile. The function returns a promise that starts pending. When the file is read successfully, the promise resolves with the file content string. Using 'await' pauses the code until the promise resolves, so the variable receives the actual content. If the file does not exist or an error occurs, the promise rejects and should be handled to avoid crashes. The example code reads 'example.txt' and prints its content to the console. The execution table traces each step from calling readFile, waiting for the promise, assigning the content, printing it, and ending the function.

Practice

1. What does fs.promises.readFile return when reading a file in Node.js?

easy

A. The file content directly as a string

B. A promise that resolves with the file content

C. A callback function to handle the file content

D. An event emitter for file reading progress

Solution

1. Step 1: Understand fs.promises.readFile behavior

   This method returns a promise that will resolve when the file is read successfully.

2. Step 2: Identify the return type

   Since it returns a promise, you can use await or .then() to get the file content asynchronously.

3. Final Answer:

   A promise that resolves with the file content -> Option B

4. Quick Check:

   fs.promises.readFile returns a promise [OK]

Hint: Remember: fs.promises methods always return promises [OK]

Common Mistakes:

• Thinking it returns file content directly
• Confusing with callback-based fs.readFile
• Expecting an event emitter

2. Which of the following is the correct syntax to read a file using fs.promises.readFile with async/await?

easy

A. const data = await fs.promises.readFile('file.txt');

B. const data = fs.promises.readFile('file.txt');

C. fs.promises.readFile('file.txt', (err, data) => {});

D. await fs.readFile('file.txt', 'utf8');

Solution

1. Step 1: Use async/await with promises

   To get the file content, you must await the promise returned by fs.promises.readFile.

2. Step 2: Check syntax correctness

   const data = await fs.promises.readFile('file.txt'); correctly uses await with fs.promises.readFile. const data = fs.promises.readFile('file.txt'); misses await, C uses callback style which is incorrect here, and D uses wrong module method.

3. Final Answer:

   const data = await fs.promises.readFile('file.txt'); -> Option A

4. Quick Check:

   Use await with fs.promises.readFile [OK]

Hint: Always await promises to get their resolved value [OK]

Common Mistakes:

• Omitting await and expecting immediate data
• Using callback style with promises API
• Mixing fs and fs.promises methods

3. What will be logged by this code snippet?

import { promises as fs } from 'fs';

async function read() {
  const content = await fs.readFile('example.txt', 'utf8');
  console.log(typeof content);
}

read();

medium

A. 'undefined'

B. 'object'

C. 'string'

D. Throws an error

Solution

1. Step 1: Understand readFile with encoding

   When you pass 'utf8' as the second argument, the promise resolves with a string containing the file content.

2. Step 2: Check the logged type

   The typeof operator on a string returns 'string', so the console logs 'string'.

3. Final Answer:

   'string' -> Option C

4. Quick Check:

   readFile with 'utf8' returns string [OK]

Hint: Add 'utf8' to get string, else Buffer is returned [OK]

Common Mistakes:

• Forgetting encoding returns Buffer, not string
• Expecting 'object' type for file content
• Not awaiting the promise before logging

4. Identify the error in this code snippet:

import { promises as fs } from 'fs';

async function readFile() {
  const data = fs.readFile('data.txt', 'utf8');
  console.log(data);
}

readFile();

medium

A. Missing await before fs.readFile call

B. Wrong import syntax for fs.promises

C. readFile does not exist in fs.promises

D. Encoding 'utf8' is invalid here

Solution

1. Step 1: Check asynchronous call handling

   The fs.readFile returns a promise, so to get the file content, you must await it.

2. Step 2: Identify the missing await

   Without await, data is a promise object, so logging it shows a promise, not file content.

3. Final Answer:

   Missing await before fs.readFile call -> Option A

4. Quick Check:

   Always await promises to get resolved value [OK]

Hint: Await promises before using their results [OK]

Common Mistakes:

• Logging promise instead of awaited result
• Confusing import syntax for fs.promises
• Passing wrong encoding string

5. You want to read multiple files ['a.txt', 'b.txt', 'c.txt'] concurrently using fs.promises.readFile and get their contents as strings. Which code snippet correctly does this?

hard

A. const contents = await fs.readFile(files, 'utf8');

B. const contents = files.map(f => await fs.readFile(f, 'utf8'));

C. const contents = files.forEach(async f => await fs.readFile(f, 'utf8'));

D. const contents = await Promise.all(files.map(f => fs.readFile(f, 'utf8')));

Solution

1. Step 1: Understand concurrent reading with Promise.all

   To read multiple files concurrently, map each filename to a promise and use Promise.all to await all results.

2. Step 2: Analyze each option

   const contents = await Promise.all(files.map(f => fs.readFile(f, 'utf8'))); correctly maps files to promises and awaits them all. const contents = files.map(f => await fs.readFile(f, 'utf8')); uses await inside map callback which is invalid syntax. const contents = files.forEach(async f => await fs.readFile(f, 'utf8')); uses forEach which returns undefined. const contents = await fs.readFile(files, 'utf8'); tries to read multiple files at once, which is invalid.

3. Final Answer:

   const contents = await Promise.all(files.map(f => fs.readFile(f, 'utf8'))); -> Option D

4. Quick Check:

   Use Promise.all with map for concurrent reads [OK]

Hint: Use Promise.all with map to await multiple promises [OK]

Common Mistakes:

• Using await inside map callback directly
• Using forEach which returns undefined
• Trying to read multiple files in one readFile call