unicorn/prefer-object-from-entries

Building an object with reduce and a spread copies the whole object on every step, so the work grows with the square of the number of items. Object.fromEntries() does it in one pass and says what the code is for.

The rule reports a reduce that starts from {} or Object.create(null). The callback must be an arrow function without a block body that returns { ...result, key: value } or Object.assign(result, { key: value }). It also reports _.fromPairs() and lodash.fromPairs().

Configure#

Off by default. To turn it on:

[lint.rules]
"unicorn/prefer-object-from-entries" = "error" # or "warn"

Every option, set to its default:

[lint.rules]
"unicorn/prefer-object-from-entries" = ["error", {
  functions = ["_.fromPairs", "lodash.fromPairs"],
}]

Examples#

const byId = users.reduce((result, user) => ({ ...result, [user.id]: user }), {});

const ages = users.reduce((result, user) => Object.assign(result, { [user.name]: user.age }), {});

const query = _.fromPairs(pairs);

const byId = Object.fromEntries(users.map(user => [user.id, user]));

const ages = Object.fromEntries(users.map(user => [user.name, user.age]));

const query = Object.fromEntries(pairs);

A reduce that starts from an object with properties, or that adds more than one property per step, isn't reported:

const settings = flags.reduce((result, flag) => ({ ...result, [flag]: true }), { debug: false });

const index = users.reduce((result, user) => ({ ...result, [user.id]: user, [user.email]: user }), {});

Options#

const query = utils.toObject(pairs);

_.fromPairs isn't in the list anymore:

const query = _.fromPairs(pairs);