Fill object in Javascript with expected properties

Question

I have a prototype

class Animal() {
    constructor(name, weight) {
        this.name = name
        this.weight = weight
    }
}

and some coming object, which contains these properties plus something else

const obj = {
    name: "name",
    weight: 5,
    someRedundantProp: "bla"
}

I really enjoy JS mapping with Object.assign which would generically create an object for me with all properties, but here I want to fill this new object only with essential fields. Surely, I can map this object like

new Animal(obj.name, obj.weight)

but if at some point I would introduce new properties I will have to change code here, what I don't want to.

Is there some better way?

"but here I want to fill this new object only with essential fields" - then define what "essential fields" are. Are they the actual properties the Animal class already "possesses"? Then declare them properly, so that you can loop over them (starting with .hasOwnProperty) to then pick out the input object properties of the same name you'd like to "copy over" — C3roe
– C3roe, Commented Nov 11, 2017 at 13:37

lumio · Accepted Answer · 2017-11-11 13:55:33Z

1

You could check if the first argument is an object and then assign the object to this.

But be warned, this has some nasty side effects too.

class Animal {
  constructor( obj ) {
    // Create an object that holds all available options
    const defaultOptions = {
      name: '',
      weight: 0,
    };
    
    // Go over all available options
    Object.keys( defaultOptions ).forEach( key => {
      // Check if obj has key, otherwise use default option
      this[ key ] = obj[ key ] || defaultOptions[ key ];
    } );
  }
}

const obj = {
  name: "name",
  weight: 5,
  someRedundantProp: "bla"
}

const a = new Animal( obj );
console.log( a );

edited Nov 11, 2017 at 13:55

answered Nov 11, 2017 at 13:46

lumio

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3 Comments

Vladyslav Zavalykhatko Over a year ago

well, in this case it will contain all properties, isn't it? including someRedundantProp

lumio Over a year ago

Yes. Oh sorry, do you only want to allow certain properties? If so, there might be another good solution available too

lumio Over a year ago

I updated my answer. It only takes valid properties now.

steliosbl · Accepted Answer · 2017-11-11 14:05:10Z

1

This should work for you:

class Animal {
    constructor(obj) {
        const defaultFields = {
            name: "",
            weight: 0
        }
        Object.keys(defaultFields).forEach(key => defaultFields[key] = obj[key]);
        Object.assign(this, defaultFields);
    }
}

edited Nov 11, 2017 at 14:05

answered Nov 11, 2017 at 13:38

steliosbl

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4 Comments

lumio Over a year ago

The reduce looks a little bit of an overkill :) Why not just using forEach and assigning the fields directly instead?

Vladyslav Zavalykhatko Over a year ago

is there a point in creating expectedFields instead of using Object.keys

steliosbl Over a year ago

@VladHatko Where are those keys specified?

Vladyslav Zavalykhatko Over a year ago

I mean, compared to my answer, without rewriting constructor

Vladyslav Zavalykhatko · Accepted Answer · 2017-11-11 13:53:43Z

0

I came to this solution for now

let animal = new Animal()
Object.keys(animal).forEach(field => animal[field] = obj[field])

answered Nov 11, 2017 at 13:53

Vladyslav Zavalykhatko

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