useExplicitLengthCheck

Diagnostic Category: lint/style/useExplicitLengthCheck

JavaScript (and super languages)

Since: v1.7.3

Note

• This rule has an unsafe fix.

Sources:

• Same as: unicorn/explicit-length-check

Enforce explicitly comparing the length, size, byteLength or byteOffset property of a value.

This rule enforces a specific style length comparisons to make them more clear.

Zero comparison examples

Section titled Zero comparison examples

Enforce comparison with === 0 when checking for zero length.

Invalid

Section titled Invalid

const isEmpty = !foo.length;

code-block.js:1:17 lint/style/useExplicitLengthCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use .length === 0 when checking .length is zero.
  
  > 1 │ const isEmpty = !foo.length;
      │                 ^^^^^^^^^^^
    2 │ 
  
  ℹ Unsafe fix: Replace .length with .length === 0
  
    1   │ - const·isEmpty·=·!foo.length;
      1 │ + const·isEmpty·=·foo.length·===·0;
    2 2 │   
  

const isEmpty = foo.length == 0;

code-block.js:1:17 lint/style/useExplicitLengthCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use .length === 0 when checking .length is zero.
  
  > 1 │ const isEmpty = foo.length == 0;
      │                 ^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Unsafe fix: Replace .length with .length === 0
  
    1 │ const·isEmpty·=·foo.length·===·0;
      │                              +   

const isEmpty = foo.length < 1;

code-block.js:1:17 lint/style/useExplicitLengthCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use .length === 0 when checking .length is zero.
  
  > 1 │ const isEmpty = foo.length < 1;
      │                 ^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Unsafe fix: Replace .length with .length === 0
  
    1   │ - const·isEmpty·=·foo.length·<·1;
      1 │ + const·isEmpty·=·foo.length·===·0;
    2 2 │   
  

const isEmpty = 0 === foo.length;

code-block.js:1:17 lint/style/useExplicitLengthCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use .length === 0 when checking .length is zero.
  
  > 1 │ const isEmpty = 0 === foo.length;
      │                 ^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Unsafe fix: Replace .length with .length === 0
  
    1   │ - const·isEmpty·=·0·===·foo.length;
      1 │ + const·isEmpty·=·foo.length·===·0;
    2 2 │   
  

const isEmpty = 0 == foo.length;

code-block.js:1:17 lint/style/useExplicitLengthCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use .length === 0 when checking .length is zero.
  
  > 1 │ const isEmpty = 0 == foo.length;
      │                 ^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Unsafe fix: Replace .length with .length === 0
  
    1   │ - const·isEmpty·=·0·==·foo.length;
      1 │ + const·isEmpty·=·foo.length·===·0;
    2 2 │   
  

const isEmpty = 1 > foo.length;

code-block.js:1:17 lint/style/useExplicitLengthCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use .length === 0 when checking .length is zero.
  
  > 1 │ const isEmpty = 1 > foo.length;
      │                 ^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Unsafe fix: Replace .length with .length === 0
  
    1   │ - const·isEmpty·=·1·>·foo.length;
      1 │ + const·isEmpty·=·foo.length·===·0;
    2 2 │   
  

// Negative style is disallowed too
const isEmpty = !(foo.length > 0);

code-block.js:2:17 lint/style/useExplicitLengthCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use .length === 0 when checking .length is zero.
  
    1 │ // Negative style is disallowed too
  > 2 │ const isEmpty = !(foo.length > 0);
      │                 ^^^^^^^^^^^^^^^^^
    3 │ 
  
  ℹ Unsafe fix: Replace .length with .length === 0
  
    1 1 │   // Negative style is disallowed too
    2   │ - const·isEmpty·=·!(foo.length·>·0);
      2 │ + const·isEmpty·=·foo.length·===·0;
    3 3 │   
  

const isEmptySet = !foo.size;

code-block.js:1:20 lint/style/useExplicitLengthCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use .size === 0 when checking .size is zero.
  
  > 1 │ const isEmptySet = !foo.size;
      │                    ^^^^^^^^^
    2 │ 
  
  ℹ Unsafe fix: Replace .size with .size === 0
  
    1   │ - const·isEmptySet·=·!foo.size;
      1 │ + const·isEmptySet·=·foo.size·===·0;
    2 2 │   
  

Valid

Section titled Valid

const isEmpty = foo.length === 0;

Non-zero comparison examples

Section titled Non-zero comparison examples

Enforce comparison with > 0 when checking for non-zero length.

Invalid

Section titled Invalid

const isNotEmpty = foo.length !== 0;

code-block.js:1:20 lint/style/useExplicitLengthCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use .length > 0 when checking .length is not zero.
  
  > 1 │ const isNotEmpty = foo.length !== 0;
      │                    ^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Unsafe fix: Replace .length with .length > 0
  
    1   │ - const·isNotEmpty·=·foo.length·!==·0;
      1 │ + const·isNotEmpty·=·foo.length·>·0;
    2 2 │   
  

const isNotEmpty = foo.length != 0;

code-block.js:1:20 lint/style/useExplicitLengthCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use .length > 0 when checking .length is not zero.
  
  > 1 │ const isNotEmpty = foo.length != 0;
      │                    ^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Unsafe fix: Replace .length with .length > 0
  
    1   │ - const·isNotEmpty·=·foo.length·!=·0;
      1 │ + const·isNotEmpty·=·foo.length·>·0;
    2 2 │   
  

const isNotEmpty = foo.length >= 1;

code-block.js:1:20 lint/style/useExplicitLengthCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use .length > 0 when checking .length is not zero.
  
  > 1 │ const isNotEmpty = foo.length >= 1;
      │                    ^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Unsafe fix: Replace .length with .length > 0
  
    1   │ - const·isNotEmpty·=·foo.length·>=·1;
      1 │ + const·isNotEmpty·=·foo.length·>·0;
    2 2 │   
  

const isNotEmpty = 0 !== foo.length;

code-block.js:1:20 lint/style/useExplicitLengthCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use .length > 0 when checking .length is not zero.
  
  > 1 │ const isNotEmpty = 0 !== foo.length;
      │                    ^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Unsafe fix: Replace .length with .length > 0
  
    1   │ - const·isNotEmpty·=·0·!==·foo.length;
      1 │ + const·isNotEmpty·=·foo.length·>·0;
    2 2 │   
  

const isNotEmpty = 0 != foo.length;

code-block.js:1:20 lint/style/useExplicitLengthCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use .length > 0 when checking .length is not zero.
  
  > 1 │ const isNotEmpty = 0 != foo.length;
      │                    ^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Unsafe fix: Replace .length with .length > 0
  
    1   │ - const·isNotEmpty·=·0·!=·foo.length;
      1 │ + const·isNotEmpty·=·foo.length·>·0;
    2 2 │   
  

const isNotEmpty = 1 <= foo.length;

code-block.js:1:20 lint/style/useExplicitLengthCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use .length > 0 when checking .length is not zero.
  
  > 1 │ const isNotEmpty = 1 <= foo.length;
      │                    ^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Unsafe fix: Replace .length with .length > 0
  
    1   │ - const·isNotEmpty·=·1·<=·foo.length;
      1 │ + const·isNotEmpty·=·foo.length·>·0;
    2 2 │   
  

const isNotEmpty = Boolean(foo.length);

code-block.js:1:20 lint/style/useExplicitLengthCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use .length > 0 when checking .length is not zero.
  
  > 1 │ const isNotEmpty = Boolean(foo.length);
      │                    ^^^^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Unsafe fix: Replace .length with .length > 0
  
    1   │ - const·isNotEmpty·=·Boolean(foo.length);
      1 │ + const·isNotEmpty·=·foo.length·>·0;
    2 2 │   
  

// Negative style is disallowed too
const isNotEmpty = !(foo.length === 0);

code-block.js:2:20 lint/style/useExplicitLengthCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use .length > 0 when checking .length is not zero.
  
    1 │ // Negative style is disallowed too
  > 2 │ const isNotEmpty = !(foo.length === 0);
      │                    ^^^^^^^^^^^^^^^^^^^
    3 │ 
  
  ℹ Unsafe fix: Replace .length with .length > 0
  
    1 1 │   // Negative style is disallowed too
    2   │ - const·isNotEmpty·=·!(foo.length·===·0);
      2 │ + const·isNotEmpty·=·foo.length·>·0;
    3 3 │   
  

if (foo.length) {}

code-block.js:1:5 lint/style/useExplicitLengthCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use .length > 0 when checking .length is not zero.
  
  > 1 │ if (foo.length) {}
      │     ^^^^^^^^^^
    2 │ 
  
  ℹ Unsafe fix: Replace .length with .length > 0
  
    1 │ if·(foo.length·>·0)·{}
      │               ++++    

const biome = foo.length ? 1 : 2

code-block.js:1:15 lint/style/useExplicitLengthCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use .length > 0 when checking .length is not zero.
  
  > 1 │ const biome = foo.length ? 1 : 2
      │               ^^^^^^^^^^
    2 │ 
  
  ℹ Unsafe fix: Replace .length with .length > 0
  
    1 │ const·biome·=·foo.length·>·0?·1·:·2
      │                          +++       

while (foo.length) {}

code-block.js:1:8 lint/style/useExplicitLengthCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use .length > 0 when checking .length is not zero.
  
  > 1 │ while (foo.length) {}
      │        ^^^^^^^^^^
    2 │ 
  
  ℹ Unsafe fix: Replace .length with .length > 0
  
    1 │ while·(foo.length·>·0)·{}
      │                  ++++    

do {} while (foo.length);

code-block.js:1:14 lint/style/useExplicitLengthCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use .length > 0 when checking .length is not zero.
  
  > 1 │ do {} while (foo.length);
      │              ^^^^^^^^^^
    2 │ 
  
  ℹ Unsafe fix: Replace .length with .length > 0
  
    1 │ do·{}·while·(·foo.length·>·0);
      │              +          ++++  

for (; foo.length; ) {};

code-block.js:1:8 lint/style/useExplicitLengthCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use .length > 0 when checking .length is not zero.
  
  > 1 │ for (; foo.length; ) {};
      │        ^^^^^^^^^^
    2 │ 
  
  ℹ Unsafe fix: Replace .length with .length > 0
  
    1 │ for·(;·foo.length·>·0;·)·{};
      │                  ++++       

Valid

Section titled Valid

const isNotEmpty = foo.length > 0;

if (foo.length > 0 || bar.length > 0) {}

Caveats

Section titled Caveats

This rule assumes that the length/size property is always numeric, even if it actually is not. In the example below the rule will trigger a warning, even though the size property is a string.

const foo1 = { size: "small" }; if (foo1.size) {}

code-block.js:1:37 lint/style/useExplicitLengthCheck  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ⚠ Use .size > 0 when checking .size is not zero.
  
  > 1 │ const foo1 = { size: “small” }; if (foo1.size) {}
      │                                     ^^^^^^^^^
    2 │ 
  
  ℹ Unsafe fix: Replace .size with .size > 0
  
    1 │ const·foo1·=·{·size:·“small”·};·if·(foo1.size·>·0)·{}
      │                                              ++++    

To properly handle this case, type inference would be required, which is not supported by Biome at the moment. We recommend disabling this rule when working with non-numeric length/size properties.

Related links

Section titled Related links

• Disable a rule
• Configure the rule fix
• Rule options