useNumericSeparators

JavaScript (and super languages)

Summary

• Rule available since: v2.0.0
• Diagnostic Category: lint/style/useNumericSeparators
• This rule isn’t recommended, so you need to enable it.
• This rule has a safe fix.
• The default severity of this rule is information.
• Sources:
  • Same as unicorn/numeric-separators-style
  • Same as unreadable_literal

How to configure

biome.json

{
  "linter": {
    "rules": {
      "style": {
        "useNumericSeparators": "error"
      }
    }
  }
}

Description

Enforce the use of numeric separators in numeric literals.

Enforces a convention of grouping digits using numeric separators. Long numbers can become difficult to read, so separating groups of digits with an underscore (_) improves code clarity. This rule also enforces proper usage of the numeric separator, by checking if the groups of digits are of the correct size.

Examples

Invalid

var a = 1234567890;

code-block.js:1:9 lint/style/useNumericSeparators  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Long numeric literal lacks separators.
  
  > 1 │ var a = 1234567890;
      │         ^^^^^^^^^^
    2 │ 
  
  ℹ Adding separators helps improve readability and clarity for long numbers.
  
  ℹ Safe fix: Add numeric separators.
  
    1   │ - var·a·=·1234567890;
      1 │ + var·a·=·1_234_567_890;
    2 2 │   
  

var a = -999_99;

code-block.js:1:10 lint/style/useNumericSeparators  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Inconsistent grouping of digits in numeric literal.
  
  > 1 │ var a = -999_99;
      │          ^^^^^^
    2 │ 
  
  ℹ Numbers with inconsistently placed separators can be misleading or confusing.
  
  ℹ Safe fix: Use consistent numeric separator grouping.
  
    1   │ - var·a·=·-999_99;
      1 │ + var·a·=·-99_999;
    2 2 │   
  

var a = 0.1234567;

code-block.js:1:9 lint/style/useNumericSeparators  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Long numeric literal lacks separators.
  
  > 1 │ var a = 0.1234567;
      │         ^^^^^^^^^
    2 │ 
  
  ℹ Adding separators helps improve readability and clarity for long numbers.
  
  ℹ Safe fix: Add numeric separators.
  
    1   │ - var·a·=·0.1234567;
      1 │ + var·a·=·0.123_456_7;
    2 2 │   
  

var a = 0b11001100;

code-block.js:1:9 lint/style/useNumericSeparators  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Long numeric literal lacks separators.
  
  > 1 │ var a = 0b11001100;
      │         ^^^^^^^^^^
    2 │ 
  
  ℹ Adding separators helps improve readability and clarity for long numbers.
  
  ℹ Safe fix: Add numeric separators.
  
    1   │ - var·a·=·0b11001100;
      1 │ + var·a·=·0b1100_1100;
    2 2 │   
  

Valid

var a = 1_234_567_890;

var a = -99_999;

var a = 0.123_456_7;

var a = 0b1100_1100;

Options

Each numeric literal type binary, octal, decimal, and hexadecimal can be configured with its own options object. Each type accepts minimumDigits or groupLength.

biome.json

{
  "linter": {
    "rules": {
      "style": {
        "useNumericSeparators": {
          "options": {
            "decimal": {
              "minimumDigits": 7,
              "groupLength": 3
            },
            "hexadecimal": {
              "minimumDigits": 4,
              "groupLength": 2
            }
          }
        }
      }
    }
  }
}

<numeric literal type>.minimumDigits

Minimum number of digits required before adding separators. For example, with minimumDigits set to 5, the number 1234 would not require separators, but 12345 would be expected to be formatted as 12_345.

Default values for minimumDigits vary by numeric literal type:

• binary: 0
• octal: 0
• decimal: 5
• hexadecimal: 0

Invalid

biome.json

{
  "linter": {
    "rules": {
      "style": {
        "useNumericSeparators": {
          "options": {
            "decimal": {
              "minimumDigits": 8
            }
          }
        }
      }
    }
  }
}

12345678;

code-block.js:1:1 lint/style/useNumericSeparators  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Long numeric literal lacks separators.
  
  > 1 │ 12345678;
      │ ^^^^^^^^
    2 │ 
  
  ℹ Adding separators helps improve readability and clarity for long numbers.
  
  ℹ Safe fix: Add numeric separators.
  
    1   │ - 12345678;
      1 │ + 12_345_678;
    2 2 │   
  

Valid

biome.json

{
  "linter": {
    "rules": {
      "style": {
        "useNumericSeparators": {
          "options": {
            "decimal": {
              "minimumDigits": 8
            }
          }
        }
      }
    }
  }
}

1234;
12_345_678;

<numeric literal type>.groupLength

Number of digits between separators. For example, with groupLength set to 2, the number 12345 would be expected to be formatted as 1_23_45.

Default values for groupLength vary by numeric literal type:

• binary: 4
• octal: 4
• decimal: 3
• hexadecimal: 2

Invalid

biome.json

{
  "linter": {
    "rules": {
      "style": {
        "useNumericSeparators": {
          "options": {
            "decimal": {
              "groupLength": 5
            }
          }
        }
      }
    }
  }
}

123_451_2345;

code-block.js:1:1 lint/style/useNumericSeparators  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Inconsistent grouping of digits in numeric literal.
  
  > 1 │ 123_451_2345;
      │ ^^^^^^^^^^^^
    2 │ 
  
  ℹ Numbers with inconsistently placed separators can be misleading or confusing.
  
  ℹ Safe fix: Use consistent numeric separator grouping.
  
    1   │ - 123_451_2345;
      1 │ + 12345_12345;
    2 2 │   
  

Valid

biome.json

{
  "linter": {
    "rules": {
      "style": {
        "useNumericSeparators": {
          "options": {
            "decimal": {
              "groupLength": 5
            }
          }
        }
      }
    }
  }
}

12345_12345;

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