CODEWITHSUNDEEP
javascript
Ben Aston
Ben Aston

Reputation: 55729

Mixins with state in JavaScript

If I understand correctly mixins can have state, whereas traits, as a subset of mixins, cannot.

How can we implement stateful mixins in JavaScript.

I can think of two ways - partial application and via closure. Are there any others?

For example:

var foo, bar, bam, closedOver = 'c';

foo = { a: function() { console.log('a'); } };
bar = { b: (function(x) { console.log('b: ' + x); }.bind(this, '1')) };
bam = { c: function() { console.log(closedOver); } };

function mix(original, ...objs) {
  objs.forEach(o => {
    Object.keys(o).forEach(k => { 
      original[k] = o[k]; 
    });
  });
}

mix(foo, bar, bam);

console.log(foo.a); // a
console.log(foo.b); // b: 1
console.log(foo.c); // c

Edit: I just realised that of course functions are objects in JavaScript and can have arbitrary state associated with them. I would like to know the answer to an ancillary question - do mixins have to be functions?

Upvotes: 0

Views: 235

Answers (2)

Peter Seliger
Peter Seliger

Reputation: 13376

How can we implement stateful mixins in JavaScript.

.

Edit: I just realised that of course functions are objects in JavaScript and can have arbitrary state associated with them. I would like to know the answer to an ancillary question - do mixins have to be functions?

Not necessarily - mixin approaches based on plain objects and [libraryName].extends are much more widespread in production than function based ones.

But especially with state, my strong opinion is that a function based approach is most natural for this languages design since it does provide delegation for free and at the same time also enables injecting of and passing around state.

I do recommend reading 2 papers, both are covering a library agnostic pure function based approach for Mixins / Traits / Talents.

  1. A fresh look at JavaScript Mixins by Angus Croll from May 2011
  2. The many talents of JavaScript for generalizing Role Oriented Programming approaches like Traits and Mixins from April 2014.

Note: the 2nd link does promote an article of mine.

Upvotes: 0

Bergi
Bergi

Reputation: 664538

Per-instance state must be created in the constructor. Therefore, mixins that need state have some kind of init method that can be called from a class constructor.

Upvotes: 2

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