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.