Default value pipe in NestJS - Step-by-Step Execution

Concept Flow - Default value pipe

Request with param

↓

Pipe intercepts param

↓

Check if param is undefined or null

Yes No↓

Replace with default

↓

Pass value to handler

↓

Handler uses value

The Default value pipe checks if a parameter is missing and replaces it with a default before the handler uses it.

Execution Sample

NestJS

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

@Injectable()
export class DefaultValuePipe implements PipeTransform {
  constructor(private readonly defaultValue: any) {}
  transform(value: any) {
    return value == null ? this.defaultValue : value;
  }
}

This pipe replaces null or undefined values with a default value before passing to the controller.

Execution Table

Step	Input Value	Check value == null	Action	Output Value
1	undefined	true	Replace with default 'guest'	guest
2	null	true	Replace with default 'guest'	guest
3	'admin'	false	Keep original	admin
4	'' (empty string)	false	Keep original
5	0	false	Keep original	0
6	false	false	Keep original	false
7	null	true	Replace with default 'guest'	guest

💡 All inputs processed; pipe replaced only null or undefined values with default.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7
value	undefined	undefined	null	'admin'	''	0	false	null
output	N/A	guest	guest	admin	''	0	false	guest

Key Moments - 2 Insights

Why does the pipe replace undefined and null but not empty string or 0?

Because the pipe checks specifically for value == null, which matches only null or undefined, not other falsy values like empty string or zero (see execution_table rows 1, 2 vs 4, 5).

What happens if the input is false? Does the pipe replace it?

No, false is a valid value and not null or undefined, so the pipe keeps it as is (see execution_table row 6).

Visual Quiz - 3 Questions

Look at the execution table, what output does the pipe produce when input is null at step 2?

A undefined

B null

C guest

D error

💡 Hint

Check the 'Output Value' column at step 2 in the execution_table.

At which step does the pipe keep the original input 'admin' without replacing it?

A Step 1

B Step 3

C Step 5

D Step 7

💡 Hint

Look for the input value 'admin' in the execution_table and see the action taken.

If the default value was changed to 'user', what would be the output at step 7?

A user

B null

C guest

D undefined

💡 Hint

Step 7 input is null, so the pipe replaces it with the default value (see variable_tracker and execution_table).

Concept Snapshot

Default Value Pipe in NestJS:
- Checks if input is null or undefined
- If yes, replaces with a default value
- Otherwise, keeps original input
- Used to ensure parameters have fallback values
- Implement by creating a class with transform method
- Helps avoid undefined errors in handlers

Full Transcript

The Default Value Pipe in NestJS intercepts incoming parameter values. It checks if the value is null or undefined. If so, it replaces the value with a preset default. Otherwise, it passes the original value unchanged. This ensures that controller handlers always receive a valid value, avoiding errors from missing parameters. The pipe is implemented as a class with a transform method that performs this check and replacement. The execution table shows how different inputs are handled step-by-step, with null and undefined replaced by the default, and other values kept as is.