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Object.entries is awesome

Jack
Typescript and React nerd. I have a soft spot for architecture and unit testing
・1 min read

Somebody asks you to loop through the properties of an object, most of us will reach for Object.keys right?

Object.keys(obj).forEach(key => {
  const value = obj[key];
  // do something
})
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Somebody asks you to map the properties of an object to other values. Object.keys to the rescue again right?

const newObj = {};
Object.keys(obj).forEach(key => {
  newObj[key] = fn(obj[key]);
});
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Wait, no reduce? That's so 2018! Let's be needlessly functional:

const newObj = Object.keys(obj).reduce((acc, key) => {
  return {
    ...acc,
    [key]: fn(obj[key])
}, {});
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Nice!

Thing is, Object.entries and Object.fromEntries are now widely supported. You can now achieve the same thing like this:

const newObj = Object.fromEntries(
  Object.entries(obj).map([key, value]) => {
    return [key, fn(value)];
  })
);
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It's also trivial to just make a utility function that combines the fromEntries and entries calls:

const mapEntries = (obj, fn) => Object.fromEntries(
  Object.entries(obj).map(fn)
);
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you could then write the above like this:

const newObj = mapEntries(obj, ([key, value]) => [key, fn(value)]);
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I love how simple this is, and you can do so much stuff that's always been a bit of a pain before.

Want to transform keys?

const newObj = mapEntries(obj, [key, value]) => [fn(key), value]);
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Want to invert an object?

const inverted = mapEntries(obj, ([key, value]) => [value, key]);
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Want to filter properties out of an object?

const filtered = Object.fromEntries(
  Object.entries(obj).filter(([ , value]) => {
    return value === true;
  })
);
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Object.entries is awesome, have a play with it.

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