useMathTrunc (JavaScript)

Language JavaScript (and super languages)

Summary

Rule available since: v2.6.3
Diagnostic Category: lint/nursery/useMathTrunc
This rule has an unsafe fix.
The default severity of this rule is warning.
Sources:
- Same as unicorn/prefer-math-trunc

How to configure

1
{
2
  "linter": {
3
    "rules": {
4
      "nursery": {
5
        "useMathTrunc": "error"
6
      }
7
    }
8
  }
9
}

Description

Require Math.trunc() instead of tricks that remove the decimal part of a number.

Some code removes the decimal part of a number with a bitwise operation, such as x | 0 or ~~x, or by turning the number into a string and parsing it back, as in parseInt(String(x), 10). These patterns are hard to read, and they give wrong results for some numbers:

Bitwise operations turn the number into a 32-bit integer first. Numbers above 2147483647 or below -2147483648 wrap around, so ~~(2 ** 32) is 0. They also turn NaN into 0.
Very large and very small numbers turn into strings such as "1e+21", which parseInt() reads as 1.

Math.trunc() removes the decimal part of any number and states the intent clearly.

This rule reports these patterns, where the bitwise operations use the number 0:

x | 0, x ^ 0, x << 0, and x >> 0
x |= 0, x ^= 0, x <<= 0, and x >>= 0
~~x
parseInt(String(x), 10) and Number.parseInt(String(x), 10)

The fix is unsafe because the result can differ from the original code. Math.trunc() keeps numbers outside the 32-bit range and keeps NaN, unlike bitwise operations. It also handles values that aren’t numbers differently from parseInt(): parseInt(String("12px"), 10) is 12, while Math.trunc("12px") is NaN.

Examples

Invalid

1
const result = value | 0;

code-block.js:1:16 lint/nursery/useMathTrunc  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use Math.trunc() instead of | 0.
  
  > 1 │ const result = value | 0;
      │                ^^^^^^^^^
    2 │ 
  
  ℹ Bitwise operations turn the number into a 32-bit integer first, so numbers above 2147483647 or below -2147483648 give wrong results.
  
  ℹ Math.trunc() removes the decimal part of any number and states the intent clearly.
  
  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 Math.trunc() instead.
  
    1   │ - const·result·=·value·|·0;
      1 │ + const·result·=·Math.trunc(value);
    2 2 │

1
const result = ~~value;

code-block.js:1:16 lint/nursery/useMathTrunc  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use Math.trunc() instead of \~\~.
  
  > 1 │ const result = \~\~value;
      │                ^^^^^^^
    2 │ 
  
  ℹ Bitwise operations turn the number into a 32-bit integer first, so numbers above 2147483647 or below -2147483648 give wrong results.
  
  ℹ Math.trunc() removes the decimal part of any number and states the intent clearly.
  
  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 Math.trunc() instead.
  
    1   │ - const·result·=·\~\~value;
      1 │ + const·result·=·Math.trunc(value);
    2 2 │

1
const result = value >> 0;

code-block.js:1:16 lint/nursery/useMathTrunc  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use Math.trunc() instead of >> 0.
  
  > 1 │ const result = value >> 0;
      │                ^^^^^^^^^^
    2 │ 
  
  ℹ Bitwise operations turn the number into a 32-bit integer first, so numbers above 2147483647 or below -2147483648 give wrong results.
  
  ℹ Math.trunc() removes the decimal part of any number and states the intent clearly.
  
  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 Math.trunc() instead.
  
    1   │ - const·result·=·value·>>·0;
      1 │ + const·result·=·Math.trunc(value);
    2 2 │

1
value |= 0;

code-block.js:1:1 lint/nursery/useMathTrunc  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use Math.trunc() instead of |= 0.
  
  > 1 │ value |= 0;
      │ ^^^^^^^^^^
    2 │ 
  
  ℹ Bitwise operations turn the number into a 32-bit integer first, so numbers above 2147483647 or below -2147483648 give wrong results.
  
  ℹ Math.trunc() removes the decimal part of any number and states the intent clearly.
  
  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 Math.trunc() instead.
  
    1   │ - value·|=·0;
      1 │ + value·=·Math.trunc(value);
    2 2 │

1
const result = parseInt(String(value), 10);

code-block.js:1:16 lint/nursery/useMathTrunc  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use Math.trunc() instead of parseInt(String(...), 10).
  
  > 1 │ const result = parseInt(String(value), 10);
      │                ^^^^^^^^^^^^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Very large and very small numbers turn into strings such as "1e+21", which parseInt() reads as 1.
  
  ℹ Math.trunc() removes the decimal part of any number and states the intent clearly.
  
  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 Math.trunc() instead.
  
    1   │ - const·result·=·parseInt(String(value),·10);
      1 │ + const·result·=·Math.trunc(value);
    2 2 │

Valid

1
const truncated = Math.trunc(value);
2
const flags = value | 1;
3
const inverted = ~value;
4
const parsed = parseInt(input, 10);