prefer-iterator-zip.md

prefer-iterator-zip

📝 Prefer Iterator.zip() over parallel-array indexing.

💼🚫 This rule is enabled in the ✅ recommended config. This rule is disabled in the ☑️ unopinionated config.

💡 This rule is manually fixable by editor suggestions.

Prefer Iterator.zip() when a loop uses an index only to read corresponding values from multiple arrays and explicitly stops at the shortest array.

Examples

// ❌
for (let index = 0; index < Math.min(names.length, scores.length); index++) {
	save(names[index], scores[index]);
}

// ✅
for (const [name, score] of Iterator.zip([names, scores])) {
	save(name, score);
}

Iterator.zip() yields corresponding values and defaults to shortest mode, stopping when any input ends. It accepts two or more inputs.

Supported patterns

The rule checks block-bodied for loops with a let index initialized to 0, incremented by one, and bounded by the shortest input using Math.min() or &&:

index < Math.min(names.length, scores.length);
index < names.length && index < scores.length;

Comparisons may be reversed. The Math.min() result may be cached in the loop initializer or an immediately preceding const or let used only by the loop condition.

Each input must be a distinct identifier read only as input[index] in the body. Arrays and typed arrays are supported; unknown inputs are assumed to be one of those. Known strings and plain objects are ignored.

Single-input bounds, callbacks, property chains, optional indexing, offsets, mutations, other input or index uses, and captured references are ignored.

Suggestions

Iterator.zip() is a newer part of the Iterator proposal than the rest, and no engine ships it yet. Node 26 has Iterator.from, Iterator.concat and the eleven prototype helpers, but Iterator.zip is undefined, so a suggestion will throw TypeError: Iterator.zip is not a function until it lands. Check the runtime supports it before accepting one.

Conversions are editor suggestions because they can change when values are read. Iterator.zip() reads each row before the body and initializes every iterator up front, while && can skip later .length reads. Review suggestions when calls or aliases can mutate inputs. Custom iteration behavior and shadowed built-ins are unsupported.

Suggestions create unique element names and preserve the body and its declarations. If a replacement would remove or relocate a comment, the rule reports without a suggestion.

Check the runtime support for Iterator.zip().