What does the @Transform() decorator do in class-transformer and when should you use it?

Q: What does the @Transform() decorator do in class-transformer and when should you use it?

import { Transform } from 'class-transformer'; export class QueryDto { // Query string '2024-01-15' -> Date instance @Transform(({ value }) => new Date(value)) @IsDate() startDate: Date; // Query string 'tag1,tag2,tag3' -> string array @Transform(({ value }) => typeof value === 'string' ? value.split(',') : value ) @IsArray() @IsString({ each: true }) tags: string[]; // Sanitize — trim and lowercase @Transform(({ value }) => value?.trim().toLowerCase()) @IsString() username: string; } @Type(() => Date) — use for straightforward class instantiation of nested objects. @Transform() — use when the conversion requires custom logic, parsing, or conditions. @Transform() also runs during serialization (instanceToPlain) — useful for formatting dates as ISO strings in responses. The transform function receives { value, key, obj, type } — the full context of the conversion. transform: true must be enabled in ValidationPipe for @Transform() to execute on incoming data.

Difficulty: 5/10

custom serialization, type conversion, nested transformation

@Transform() applies a custom transformation function when class-transformer converts an object. Use it when a type conversion is too complex for @Type() alone — such as splitting a comma-separated string into an array, parsing a date string into a Date instance, or sanitizing a value by trimming and lowercasing.

@Transform() for dates, arrays, and sanitization

When to use @Transform() vs @Type():

1
@Type(() => Date) — use for straightforward class instantiation of nested objects.
2
@Transform() — use when the conversion requires custom logic, parsing, or conditions.
3
@Transform() also runs during serialization (instanceToPlain) — useful for formatting dates as ISO strings in responses.
4
The transform function receives { value, key, obj, type } — the full context of the conversion.
5
transform: true must be enabled in ValidationPipe for @Transform() to execute on incoming data.

Scenario Questions

0-2 years experience

Follow-up Questions