prefer-array-from-map.md

prefer-array-from-map

📝 Prefer using the Array.from() mapping function argument.

💼 This rule is enabled in the following configs: ✅ recommended, ☑️ unopinionated.

🔧💡 This rule is automatically fixable by the --fix CLI option and manually fixable by editor suggestions.

Prefer passing the mapping function directly to Array.from() instead of calling .map() on the created array or filling an empty array in a for…of loop.

Array.from(iterable, mapFunction) avoids creating an intermediate array before mapping.

When checking .map() calls, this rule only reports inline arrow function callbacks that do not use Array#map's third callback argument.

When checking for…of loops, this rule only reports adjacent empty-array declarations followed by a simple loop body that adds exactly one mapped value. Trivial copies are handled by prefer-spread.

Examples

// ❌
const foo = Array.from(bar).map(element => element.id);

// ✅
const foo = Array.from(bar, element => element.id);

// ❌
const foo = Array.from(bar).map((element, index) => `${index}:${element}`);

// ✅
const foo = Array.from(bar, (element, index) => `${index}:${element}`);

// ❌
const foo = [];
for (const element of bar) {
	foo.push(element.id);
}

// ✅
const foo = Array.from(bar, element => element.id);

// ❌
const foo = [];
for (const [index, element] of bar.entries()) {
	foo.push(`${index}:${element}`);
}

// ✅
const foo = Array.from(bar.entries(), ([index, element]) => `${index}:${element}`);

// ✅
const foo = Array.from(bar).map((element, index, array) => {
	return array.includes(element);
});

// ✅
const foo = [];
for (const element of bar) {
	foo.push(element);
}