File validation pipe in NestJS - Step-by-Step Execution

Concept Flow - File validation pipe

File Upload Request

↓

Pipe Receives File

↓

Validate File Type?

No→Throw Error: Invalid Type

Yes↓

Validate File Size?

No→Throw Error: File Too Large

Yes↓

Pass Valid File to Controller

↓

Controller Processes File

The file validation pipe checks the uploaded file's type and size before allowing it to reach the controller.

Execution Sample

NestJS

import { PipeTransform, Injectable, BadRequestException } from '@nestjs/common';

@Injectable()
export class FileValidationPipe implements PipeTransform {
  transform(file: Express.Multer.File) {
    if (!file) throw new BadRequestException('No file uploaded');
    if (!['image/png', 'image/jpeg'].includes(file.mimetype)) {
      throw new BadRequestException('Invalid file type');
    }
    if (file.size > 1024 * 1024) {
      throw new BadRequestException('File too large');
    }
    return file;
  }
}

This pipe checks if a file exists, then validates its type and size, throwing errors if invalid.

Execution Table

Step	Action	Input File	Check	Result	Next Step
1	Receive file	{mimetype: 'image/png', size: 500000}	File exists?	Yes	Validate type
2	Validate type	image/png	Is type in ['image/png', 'image/jpeg']?	Yes	Validate size
3	Validate size	500000 bytes	Is size <= 1MB?	Yes	Return file
4	Return	Valid file	N/A	File passed validation	Controller receives file
5	Receive file	null	File exists?	No	Throw error: No file uploaded
6	Receive file	{mimetype: 'application/pdf', size: 500000}	File exists?	Yes	Validate type
7	Validate type	application/pdf	Is type in ['image/png', 'image/jpeg']?	No	Throw error: Invalid file type
8	Receive file	{mimetype: 'image/jpeg', size: 2000000}	File exists?	Yes	Validate type
9	Validate type	image/jpeg	Is type in ['image/png', 'image/jpeg']?	Yes	Validate size
10	Validate size	2000000 bytes	Is size <= 1MB?	No	Throw error: File too large

💡 Execution stops when file fails validation or passes all checks successfully.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
file	undefined	{mimetype: 'image/png', size: 500000}	{mimetype: 'image/png', size: 500000}	{mimetype: 'image/png', size: 500000}	Returned valid file
file	undefined	null	null	null	Error thrown: No file uploaded
file	undefined	{mimetype: 'application/pdf', size: 500000}	{mimetype: 'application/pdf', size: 500000}	N/A	Error thrown: Invalid file type
file	undefined	{mimetype: 'image/jpeg', size: 2000000}	{mimetype: 'image/jpeg', size: 2000000}	{mimetype: 'image/jpeg', size: 2000000}	Error thrown: File too large

Key Moments - 3 Insights

Why does the pipe throw an error if the file is null?

Because the pipe first checks if a file exists (Step 1 in execution_table). If the file is null or undefined, it immediately throws a 'No file uploaded' error to prevent further processing.

What happens if the file type is not in the allowed list?

At Step 7 in execution_table, if the file mimetype is not 'image/png' or 'image/jpeg', the pipe throws an 'Invalid file type' error and stops execution.

How does the pipe handle files larger than 1MB?

At Step 10, the pipe checks the file size. If it exceeds 1MB (1024*1024 bytes), it throws a 'File too large' error, preventing the file from reaching the controller.

Visual Quiz - 3 Questions

Look at the execution table, what is the result at Step 3 when the file size is 500000 bytes?

A File size is valid, proceed to return file

B File size is too large, throw error

C File type is invalid, throw error

D No file uploaded error

💡 Hint

Check Step 3 row in execution_table for file size validation result.

At which step does the pipe throw an error for an unsupported file type?

A Step 10

B Step 2

C Step 7

D Step 5

💡 Hint

Look for the step where file mimetype is checked against allowed types and error is thrown.

If the file is missing, what will the variable 'file' be at Step 1 and what happens next?

A 'file' is valid, pipe continues validation

B 'file' is null, pipe throws 'No file uploaded' error

C 'file' is undefined, pipe returns file

D 'file' is null, pipe accepts file

💡 Hint

Refer to variable_tracker and execution_table Step 5 for file null check.

Concept Snapshot

File Validation Pipe in NestJS:
- Implements PipeTransform interface
- Checks if file exists
- Validates file mimetype against allowed list
- Validates file size limit
- Throws BadRequestException on failure
- Passes valid file to controller
- Ensures safe file uploads

Full Transcript

This visual execution trace shows how a NestJS file validation pipe works step-by-step. When a file upload request arrives, the pipe first checks if a file is present. If not, it throws a 'No file uploaded' error. If a file exists, it checks the file type against allowed types like 'image/png' and 'image/jpeg'. If the type is invalid, it throws an error. Next, it checks the file size to ensure it does not exceed 1MB. If the file is too large, it throws an error. If all checks pass, the pipe returns the valid file to the controller for further processing. The variable tracker shows how the file variable changes or triggers errors at each step. Key moments clarify common confusions about missing files, invalid types, and size limits. The quiz questions help reinforce understanding by referencing specific steps and variable states in the execution table.