useReduceTypeParameter

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TypeScript and TSX

Summary

Rule available since: v2.4.12
Diagnostic Category: lint/nursery/useReduceTypeParameter
This rule has an unsafe fix.
The default severity of this rule is information.
Sources:
- Inspired from @typescript-eslint/prefer-reduce-type-parameter

How to configure

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{
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  "linter": {
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    "rules": {
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      "nursery": {
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        "useReduceTypeParameter": "error"
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      }
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    }
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  }
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}

Description

Enforce using a type parameter on Array#reduce instead of casting the initial value.

When using Array#reduce, the type of the accumulator is inferred from the initial value. If you use a type assertion (as or angle bracket <T>) on the initial value, the type is not checked against the accumulator usage in the callback. Using a type parameter on reduce instead is more type-safe because TypeScript will verify that the callback’s return type matches the declared type.

Examples

Invalid

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const arr: number[] = [1, 2, 3];
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arr.reduce((sum, num) => sum.concat(num * 2), [] as number[]);

code-block.ts:2:1 lint/nursery/useReduceTypeParameter  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ The initial value of Array#reduce uses a type assertion.
  
    1 │ const arr: number[] = [1, 2, 3];
  > 2 │ arr.reduce((sum, num) => sum.concat(num * 2), [] as number[]);
      │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    3 │ 
  
  ℹ Type assertions can hide type mismatches in the reducer callback. Use a type parameter on the call instead, so TypeScript checks the return type.
  
  ℹ This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.
  
  ℹ Unsafe fix: Use a type parameter instead of type assertion.
  
    1 1 │   const arr: number[] = [1, 2, 3];
    2   │ - arr.reduce((sum,·num)·=>·sum.concat(num·*·2),·[]·as·number[]);
      2 │ + arr.reduce<number[]>((sum,·num)·=>·sum.concat(num·*·2),·[]);
    3 3 │

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const arr: string[] = ['a', 'b'];
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arr.reduce((acc, name) => ({ ...acc, [name]: true }), {} as Record<string, boolean>);

code-block.ts:2:1 lint/nursery/useReduceTypeParameter  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ The initial value of Array#reduce uses a type assertion.
  
    1 │ const arr: string[] = [‘a’, ‘b’];
  > 2 │ arr.reduce((acc, name) => ({ …acc, [name]: true }), {} as Record<string, boolean>);
      │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    3 │ 
  
  ℹ Type assertions can hide type mismatches in the reducer callback. Use a type parameter on the call instead, so TypeScript checks the return type.
  
  ℹ This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.
  
  ℹ Unsafe fix: Use a type parameter instead of type assertion.
  
    1 1 │   const arr: string[] = [‘a’, ‘b’];
    2   │ - arr.reduce((acc,·name)·=>·({·...acc,·[name]:·true·}),·{}·as·Record<string,·boolean>);
      2 │ + arr.reduce<Record<string,·boolean>>((acc,·name)·=>·({·...acc,·[name]:·true·}),·{});
    3 3 │

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const arr: number[] = [1, 2, 3];
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arr.reduceRight((sum, num) => sum.concat(num * 2), [] as number[]);

code-block.ts:2:1 lint/nursery/useReduceTypeParameter  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ The initial value of Array#reduceRight uses a type assertion.
  
    1 │ const arr: number[] = [1, 2, 3];
  > 2 │ arr.reduceRight((sum, num) => sum.concat(num * 2), [] as number[]);
      │ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    3 │ 
  
  ℹ Type assertions can hide type mismatches in the reducer callback. Use a type parameter on the call instead, so TypeScript checks the return type.
  
  ℹ This rule belongs to the nursery group, which means it is not yet stable and may change in the future. Visit https://biomejs.dev/linter/#nursery for more information.
  
  ℹ Unsafe fix: Use a type parameter instead of type assertion.
  
    1 1 │   const arr: number[] = [1, 2, 3];
    2   │ - arr.reduceRight((sum,·num)·=>·sum.concat(num·*·2),·[]·as·number[]);
      2 │ + arr.reduceRight<number[]>((sum,·num)·=>·sum.concat(num·*·2),·[]);
    3 3 │

Valid

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const arr: number[] = [1, 2, 3];
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arr.reduce<number[]>((sum, num) => sum.concat(num * 2), []);
3

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arr.reduce((a, b) => a + b);
5

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arr.reduce((sum, n) => sum + n, 0);