unicorn/no-useless-spread Correctness

✅ This rule is turned on by default.

🛠️ An auto-fix is available for this rule for some violations.

What it does

Disallows using spread syntax in following, unnecessary cases:

• Spread an array literal as elements of an array literal
• Spread an array literal as arguments of a call or a new call
• Spread an object literal as properties of an object literal
• Use spread syntax to clone an array created inline

Why is this bad?

• The following builtins accept an iterable, so it's unnecessary to convert the iterable to an array:

  • Map constructor
  • WeakMap constructor
  • Set constructor
  • WeakSet constructor
  • TypedArray constructor
  • Array.from(…)
  • TypedArray.from(…)
  • Promise.{all,allSettled,any,race}(…)
  • Object.fromEntries(…)

• for…of loop can iterate over any iterable object not just array, so it's unnecessary to convert the iterable to an array.

• yield* can delegate to another iterable, so it's unnecessary to convert the iterable to an array.

Example

const array = [firstElement, ...[secondElement], thirdElement];
const object = { firstProperty, ...{ secondProperty }, thirdProperty };
foo(firstArgument, ...[secondArgument], thirdArgument);
const object = new Foo(firstArgument, ...[secondArgument], thirdArgument);
const set = new Set([...iterable]);
async function foo() {
  const results = await Promise.all([...iterable]);
}
for (const foo of [...set]);
function* foo() {
  yield* [...anotherGenerator()];
}
function foo(bar) {
  return [...bar.map((x) => x * 2)];
}

// Pass

const array = [firstElement, secondElement, thirdElement];
const object = { firstProperty, secondProperty, thirdProperty };
foo(firstArgument, secondArgument, thirdArgument);
const object = new Foo(firstArgument, secondArgument, thirdArgument);
const array = [...foo, bar];
const object = { ...foo, bar };
foo(foo, ...bar);
const object = new Foo(...foo, bar);
const set = new Set(iterable);
async function foo() {
  const results = await Promise.all(iterable);
}
for (const foo of set);
function* foo() {
  yield* anotherGenerator();
}
function foo(bar) {
  return bar.map((x) => x * 2);
}

References

• Rule Source