noObjectLiteralComparison (JavaScript)

Language JavaScript (and super languages)

Summary

Rule available since: v2.6.5
Diagnostic Category: lint/nursery/noObjectLiteralComparison
This rule doesn’t have a fix.
The default severity of this rule is error.
Sources:
- Same as oxc/bad-object-literal-comparison

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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        "noObjectLiteralComparison": "error"
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      }
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    }
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  }
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}

Description

Disallow comparing a value with an empty object {} or an empty array [].

Code like value === {} or list === [] looks like it checks whether a value is empty, but it doesn’t. Each {} and [] creates a new object, and JavaScript compares objects and arrays by reference, not by their contents. With ===, the comparison is always false, and with !==, it’s always true. With == and !=, JavaScript first converts {} to the string "[object Object]" and [] to the empty string "" when the other value isn’t an object, so "" == [] and 0 == [] are both true, even though neither value is an empty array.

To check whether an object has no properties, use Object.keys(value).length === 0. To check whether an array is empty, use value.length === 0.

Examples

Invalid

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if (value === {}) {}

code-block.js:1:5 lint/nursery/noObjectLiteralComparison ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ This compares a value with an empty object literal.
  
  > 1 │ if (value === {}) {}
      │     ^^^^^^^^^^^^
    2 │ 
  
  ℹ Each object literal creates a new object. Comparing a value with it never checks whether the value is empty: objects and arrays are compared by reference, not by their contents.
  
  ℹ To check whether an object has no properties, use Object.keys(value).length === 0.
  
  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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if (list !== []) {}

code-block.js:1:5 lint/nursery/noObjectLiteralComparison ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ This compares a value with an empty array literal.
  
  > 1 │ if (list !== []) {}
      │     ^^^^^^^^^^^
    2 │ 
  
  ℹ Each array literal creates a new array. Comparing a value with it never checks whether the value is empty: objects and arrays are compared by reference, not by their contents.
  
  ℹ To check whether an array is empty, use value.length === 0.
  
  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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if (value == {}) {}

code-block.js:1:5 lint/nursery/noObjectLiteralComparison ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ This compares a value with an empty object literal.
  
  > 1 │ if (value == {}) {}
      │     ^^^^^^^^^^^
    2 │ 
  
  ℹ Each object literal creates a new object. Comparing a value with it never checks whether the value is empty: objects and arrays are compared by reference, not by their contents.
  
  ℹ To check whether an object has no properties, use Object.keys(value).length === 0.
  
  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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if (value !== null && typeof value === "object" && Object.keys(value).length === 0) {}
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if (Array.isArray(list) && list.length === 0) {}
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if (value === null) {}