map function for objects (instead of arrays)

Question

I have an object:

myObject = { \'a\': 1, \'b\': 2, \'c\': 3 }

I am looking for a native method, similar to Array.prototype.map

Answer 1

The accepted answer has two drawbacks:

• It misuses Array.prototype.reduce, because reducing means to change the structure of a composite type, which doesn't happen in this case.
• It is not particularly reusable

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An ES6/ES2015 functional approach

Please note that all functions are defined in curried form.

// small, reusable auxiliary functions

const keys = o => Object.keys(o);

const assign = (...o) => Object.assign({}, ...o);

const map = f => xs => xs.map(x => f(x));

const mul = y => x => x * y;

const sqr = x => mul(x) (x);


// the actual map function

const omap = f => o => {
  o = assign(o); // A
  map(x => o[x] = f(o[x])) (keys(o)); // B
  return o;
};


// mock data

const o = {"a":1, "b":2, "c":3};


// and run

console.log(omap(sqr) (o));
console.log(omap(mul(10)) (o));

• In line A o is reassigned. Since Javascript passes reference values by sharing, a shallow copy of o is generated. We are now able to mutate o within omap without mutating o in the parent scope.
• In line B map's return value is ignored, because map performs a mutation of o. Since this side effect remains within omap and isn't visible in the parent scope, it is totally acceptable.

This is not the fastest solution, but a declarative and reusable one. Here is the same implementation as a one-line, succinct but less readable:

const omap = f => o => (o = assign(o), map(x => o[x] = f(o[x])) (keys(o)), o);

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Addendum - why are objects not iterable by default?

ES2015 specified the iterator and iterable protocols. But objects are still not iterable and thus not mappable. The reason is the mixing of data and program level.