noAccumulatingSpread

Diagnostic Category: lint/performance/noAccumulatingSpread

JavaScript (and super languages)

Since: v1.0.0

Description

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

Invalid

var a = ['a', 'b', 'c'];
a.reduce((acc, val) => [...acc, val], []);

code-block.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];}, []);

code-block.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}), {});

code-block.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

var a = ['a', 'b', 'c'];
a.reduce((acc, val) => {acc.push(val); return acc}, []);

How to configure

{
  "linter": {
    "rules": {
      "performance": {
        "noAccumulatingSpread": "error"
      }
    }
  }
}