noAccumulatingSpread (since v1.0.0)
Diagnostic Category: lint/performance/noAccumulatingSpread
Disallow the use of spread (...) syntax on accumulators.
Spread syntax allows an iterable to be expanded into its individual elements.
Spread syntax should be avoided on accumulators (like those in .reduce)
because it causes a time complexity of O(n^2) instead of O(n).
Source: https://prateeksurana.me/blog/why-using-object-spread-with-reduce-bad-idea/
Examples
Section titled ExamplesInvalid
Section titled Invalidvar a = ['a', 'b', 'c'];a.reduce((acc, val) => [...acc, val], []);performance/noAccumulatingSpread.js:2:25 lint/performance/noAccumulatingSpread ━━━━━━━━━━━━━━━━━━━━━
✖ Avoid the use of spread (`...`) syntax on accumulators.
1 │ var a = ['a', 'b', 'c'];
> 2 │ a.reduce((acc, val) => [...acc, val], []);
│ ^^^^^^
3 │
ℹ Spread syntax should be avoided on accumulators (like those in `.reduce`) because it causes a time complexity of `O(n^2)`.
ℹ Consider methods such as .splice or .push instead.
var a = ['a', 'b', 'c'];a.reduce((acc, val) => {return [...acc, val];}, []);performance/noAccumulatingSpread.js:2:33 lint/performance/noAccumulatingSpread ━━━━━━━━━━━━━━━━━━━━━
✖ Avoid the use of spread (`...`) syntax on accumulators.
1 │ var a = ['a', 'b', 'c'];
> 2 │ a.reduce((acc, val) => {return [...acc, val];}, []);
│ ^^^^^^
3 │
ℹ Spread syntax should be avoided on accumulators (like those in `.reduce`) because it causes a time complexity of `O(n^2)`.
ℹ Consider methods such as .splice or .push instead.
var a = ['a', 'b', 'c'];a.reduce((acc, val) => ({...acc, [val]: val}), {});performance/noAccumulatingSpread.js:2:26 lint/performance/noAccumulatingSpread ━━━━━━━━━━━━━━━━━━━━━
✖ Avoid the use of spread (`...`) syntax on accumulators.
1 │ var a = ['a', 'b', 'c'];
> 2 │ a.reduce((acc, val) => ({...acc, [val]: val}), {});
│ ^^^^^^
3 │
ℹ Spread syntax should be avoided on accumulators (like those in `.reduce`) because it causes a time complexity of `O(n^2)`.
ℹ Consider methods such as .splice or .push instead.
Valid
Section titled Validvar a = ['a', 'b', 'c'];a.reduce((acc, val) => {acc.push(val); return acc}, []);