Setting Class values in constructor to parameters

Question

I have this code, which is kinda long winded but gets there in the end …

class WorksButLongWinded {
  constructor (callback, params = {}) {
    // defaults ...
    this.foo = "string";
    this.bar = 200;

    this.callback = callback;

    // params ...
    if( typeof params.foo != "undefined" ) { this.foo = params.foo }
    if( typeof params.bar != "undefined" ) { this.bar = params.bar }
  }
}

I thought I might try using destructuring to speed things along, like this:

class DoesNotWork {
  constructor (callback, params = {}) {
    {
      this.foo = "string", 
      this.bar = 200
    } = params;

    this.callback = callback;

  }
}

... except that doesn't work. It doesn't even pass syntax muster.

What is a clean and simple way of writing a class constructor that takes an optional params object with various optional parameters that override some default values?

@ShubhamGupta umm ... yes? I think that's what it is called. — Erics
– Erics, Commented Sep 24, 2018 at 5:31

Derek 朕會功夫 · Accepted Answer · 2018-09-24 05:36:36Z

2

You mean something like this?

class WorksButLongWinded {
  constructor(callback, params) {
    let defaults = {
      foo: "string",
      bar: 200
    };

    this.callback = callback;
    Object.assign(this, defaults, params);
  }
}

let a = new WorksButLongWinded(() => {});
console.log(a.foo);  // "string"
console.log(a.bar);  // "200"

let b = new WorksButLongWinded(() => {}, {foo: "bar"});
console.log(b.foo);  // "bar"
console.log(b.bar);  // "200"

Object.assign does exactly what you want.

edited Sep 24, 2018 at 5:36

answered Sep 24, 2018 at 5:32

Derek 朕會功夫

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2 Comments

Erics Over a year ago

ooh, Object.assign looks useful

trk Over a year ago

I like this solution. I just learnt .assign() could save a lot of typing like Object.keys().forEach(...). An approach I was going to post.

Shubham Gupta · Accepted Answer · 2018-09-24 05:31:46Z

1

I think the way you are destructuring is incorrect. Try the below code where we destructure from params and then assign the values to this.

class WorksButLongWinded {
  constructor (callback, params = {}) {
    const {foo = "string", bar = 200} = params;
    this.foo = foo;
    this.bar = bar;
    this.callback = callback;
  }
}

answered Sep 24, 2018 at 5:31

Shubham Gupta

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2 Comments

Erics Over a year ago

OK, that is shorter (coding the defaults into the destructure and then assigning those values)

Shubham Gupta Over a year ago

@Erics yes that way it is cleaner as well.

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