noMisusedSpread (JavaScript)

Language JavaScript (and super languages)

Summary

Rule available since: v2.6.8
Diagnostic Category: lint/nursery/noMisusedSpread
This rule doesn’t have a fix.
The default severity of this rule is information.
This rule belongs to the following domains:
- types
Sources:
- Same as @typescript-eslint/no-misused-spread

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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        "noMisusedSpread": "error"
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      }
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    }
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  }
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}

Description

Disallow spreading values in ways that can cause unexpected results.

The spread syntax (...) copies values into a new array, object, or list of arguments. For some values the copy doesn’t contain what you might expect:

Spreading a string into an array, as in [...text], splits it into Unicode code points. Characters made of several code points, such as many emoji, end up split into separate pieces.
Spreading a Promise into an object copies nothing. The value it resolves to is only available after awaiting it.
Spreading a function into an object copies nothing, unless the function has properties of its own.
Spreading a Map into an object creates an empty object, because the entries of a map aren’t properties.
Spreading an array into an object creates properties named after its indexes, such as 0 and 1.
Spreading another iterable value, such as a Set, into an object doesn’t copy the values it contains.
Spreading a class instance into an object creates a plain object without the methods and getters of the class.
Spreading a class into an object only copies its static properties.

This rule uses type information to find these values.

Examples

Invalid

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declare const text: string;
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const characters = [...text];

/invalid-string.ts:2:24 lint/nursery/noMisusedSpread ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Spreading a string splits it into code points, which can break characters apart.
  
    1 │ declare const text: string;
  > 2 │ const characters = [...text];
      │                        ^^^^
    3 │ 
  
  ℹ Characters made of several code points, such as many emoji and some letters with accents, are split into separate pieces.
  
  ℹ Use Intl.Segmenter to split the string into the characters that a reader sees. If the string only contains simple characters, suppress this diagnostic.
  
  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.

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declare function getUser(): Promise<{ name: string }>;
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const user = { ...getUser() };

/invalid-promise.ts:2:19 lint/nursery/noMisusedSpread ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Spreading a Promise into an object copies nothing.
  
    1 │ declare function getUser(): Promise<{ name: string }>;
  > 2 │ const user = { ...getUser() };
      │                   ^^^^^^^^^
    3 │ 
  
  ℹ The value of a Promise is only available after it resolves.
  
  ℹ Did you forget to await it?
  
  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.

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declare const scores: Map<string, number>;
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const copy = { ...scores };

/invalid-map.ts:2:19 lint/nursery/noMisusedSpread ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Spreading a map into an object creates an empty object.
  
    1 │ declare const scores: Map<string, number>;
  > 2 │ const copy = { ...scores };
      │                   ^^^^^^
    3 │ 
  
  ℹ The entries of a map aren't properties, so they aren't copied.
  
  ℹ To turn the entries of a Map into properties, use Object.fromEntries().
  
  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.

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class Point {
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    constructor(public x: number, public y: number) {}
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    length() {
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        return Math.hypot(this.x, this.y);
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    }
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}
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const copy = { ...new Point(1, 2) };

/invalid-class-instance.ts:7:19 lint/nursery/noMisusedSpread ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Spreading a class instance into an object loses the methods of its class.
  
    5 │     }
    6 │ }
  > 7 │ const copy = { ...new Point(1, 2) };
      │                   ^^^^^^^^^^^^^^^
    8 │ 
  
  ℹ Only the own properties of the instance are copied into a plain object. Methods and getters of the class aren't.
  
  ℹ Keep using the instance, or create a new one with the class.
  
  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.

Valid

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declare const text: string;
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const words = text.split(" ");
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declare const items: number[];
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const copy = [...items];
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declare function getUser(): Promise<{ name: string }>;
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async function loadUser() {
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    return { ...(await getUser()) };
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}
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declare const scores: Map<string, number>;
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const entries = Object.fromEntries(scores);