Class-transformer: feat: use default value for property when doing plainToClass

Created on 27 Feb 2018 · 16Comments · Source: typestack/class-transformer

I have a class:

class A {
  a: string = '123'
}

When I do plainToClass(A, {}) it returns me an object where a is undefined.

What I expect is to be able to provide default value to properties. It can be whatever syntax, not necessary the one that I shown. E.g if I do just new A() then I will have the desired a === '123'.

Maybe there is some decorator or other way that I'm missing?

transformations needs docs donreleased feature

Source

kirillgroshkov

👍19

Most helpful comment

IMO, this is something that should happen by default. If there's a default value set on the target class, don't override it with undefined. This is standard behaviour in serialisation libraries I've worked with in the past.
In the meantime, you could work around it with the Transform decorator.

class A {
  @Transform(value => value || '123', { toClassOnly: true }) 
  a: string
}

guscastro on 28 Feb 2018

👍10

All 16 comments

class A {
  @Transform(value => value || '123', { toClassOnly: true }) 
  a: string
}

guscastro on 28 Feb 2018

👍10

@guscastro Thanks for the workaround, I think it will work for my case!

I agree that what I described is kind of default behaviour that everyone would expect. So, would be nice to maybe have a specific decorator for that, e.g

@Default('123')
a: string

kirillgroshkov on 28 Feb 2018

👍9 👎1

Came also to realisation there wasn't a decorator for this.
@pleerock I agree with @kirillgroshkov that would be a nice to have.
Could we reopen this ticket please?

nolazybits on 24 Mar 2018

👍1

Yeah, we can have a @Default decorator. This needs co-op with class-validator.

NoNameProvided on 28 Mar 2018

👍5 👎1

Is the workaround given by @guscastro supposed to work ?
model.a is still undefined on my side.

const json = {};
class A {
    @Transform(value => value || "123", { toClassOnly: true })
    a: string;
}
model = plainToClass(A, json);

What am I missing ?

krazar on 29 Apr 2018

Ok, it seems that I have to add the @Exposed decorator to the field for it to be picked as key.
Is it supposed to be the default behavior ?

krazar on 30 Apr 2018

👍1

Any update on this one? Would be lovely to have @Default decorator...

otomeskhy on 19 Jun 2018

👍8

Here is the decorator I'm using:

import { Transform } from 'class-transformer';

export function Default(defaultValue: any) {
  return Transform((value: any) => (value !== null && value !== undefined ? value : defaultValue));
}

jbdemonte on 1 Oct 2018

👍3 ❤1

The solution should be as @guscastro noted:

IMO, this is something that should happen by default. If there's a default value set on the target class, don't override it with undefined.

NoNameProvided on 26 Oct 2018

👍8

Vote for this. There's a corresponding issue in class-validator (https://github.com/typestack/class-validator/issues/177)

zen0wu on 13 Jun 2019

@jbdemonte 's solution has an issue when used with non-simple data types.
For example if using @Default([]) to initiate an array, all instances would share the same array.
To prevent this, I extended his solution to (flatly) copy array and object values, or to use a function returning some value.

export function Default(defaultValue: any) {
    return Transform((value: any) => {
        if (value !== null && value !== undefined) return value;
        if(typeof defaultValue==='function') return defaultValue();
        if(Array.isArray(defaultValue)) return [...defaultValue];
        if(typeof defaultValue==='object') {
            return (defaultValue===null) ? null : {...defaultValue};
        }
        return defaultValue;
    });
}

@Default([])
@Default({})
@Default(()=>return {...data...} as SomeComplexDefaultValue )

jbjhjm on 14 Feb 2020

👍1

If you already have lodash installed, @jbjhjm's answer can be written as:

import { Transform } from 'class-transformer'
import { cloneDeep } from 'lodash'

export function Default(defaultValue: unknown): PropertyDecorator {
  return Transform((value: unknown) => value ?? cloneDeep(defaultValue))
}