useValidCharacterComparison (JavaScript)

Language JavaScript (and super languages)

Summary

Diagnostic Category: lint/nursery/useValidCharacterComparison
This rule doesn’t have a fix.
The default severity of this rule is error.
Sources:
- Same as oxc/bad-char-at-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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        "useValidCharacterComparison": "error"
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      }
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    }
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  }
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}

Description

Disallow comparing a single character with a string that has more than one character.

charAt() returns a string that has one character at most. The same is true when you read a character from a string with at() or square brackets, like text[0]. Comparing that result with a longer string with ===, ==, !==, or != always gives the same answer, so the comparison is likely a mistake.

JavaScript counts some characters, such as most emojis, as two characters in a string. For example, "😀".length is 2, so text.charAt(0) can never be equal to "😀".

Arrays also have at() and square-bracket access, and their items can be longer strings. For this reason, the rule checks at() and square brackets only when the value is clearly a string: a string literal, or a variable declared with the string type or with a const string literal.

Examples

Invalid

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text.charAt(4) === "a2";

code-block.js:1:1 lint/nursery/useValidCharacterComparison ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ This comparison is always false.
  
  > 1 │ text.charAt(4) === "a2";
      │ ^^^^^^^^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ This expression returns one character at most.
  
  > 1 │ text.charAt(4) === "a2";
      │ ^^^^^^^^^^^^^^
    2 │ 
  
  ℹ But this string has more than one character.
  
  > 1 │ text.charAt(4) === "a2";
      │                    ^^^^
    2 │ 
  
  ℹ Compare the character with a string of one character, or use a method such as startsWith() to check for longer text.
  
  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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text.charAt(0) !== "/n";

code-block.js:1:1 lint/nursery/useValidCharacterComparison ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ This comparison is always true.
  
  > 1 │ text.charAt(0) !== "/n";
      │ ^^^^^^^^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ This expression returns one character at most.
  
  > 1 │ text.charAt(0) !== "/n";
      │ ^^^^^^^^^^^^^^
    2 │ 
  
  ℹ But this string has more than one character.
  
  > 1 │ text.charAt(0) !== "/n";
      │                    ^^^^
    2 │ 
  
  ℹ Compare the character with a string of one character, or use a method such as startsWith() to check for longer text.
  
  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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"abc".at(0) === "ab";

code-block.js:1:1 lint/nursery/useValidCharacterComparison ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ This comparison is always false.
  
  > 1 │ "abc".at(0) === "ab";
      │ ^^^^^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ This expression returns one character at most.
  
  > 1 │ "abc".at(0) === "ab";
      │ ^^^^^^^^^^^
    2 │ 
  
  ℹ But this string has more than one character.
  
  > 1 │ "abc".at(0) === "ab";
      │                 ^^^^
    2 │ 
  
  ℹ Compare the character with a string of one character, or use a method such as startsWith() to check for longer text.
  
  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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const text = "abc";
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text[0] === "ab";

code-block.js:2:1 lint/nursery/useValidCharacterComparison ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ✖ This comparison is always false.
  
    1 │ const text = "abc";
  > 2 │ text[0] === "ab";
      │ ^^^^^^^^^^^^^^^^
    3 │ 
  
  ℹ This expression returns one character at most.
  
    1 │ const text = "abc";
  > 2 │ text[0] === "ab";
      │ ^^^^^^^
    3 │ 
  
  ℹ But this string has more than one character.
  
    1 │ const text = "abc";
  > 2 │ text[0] === "ab";
      │             ^^^^
    3 │ 
  
  ℹ Compare the character with a string of one character, or use a method such as startsWith() to check for longer text.
  
  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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text.charAt(4) === "a";
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text.charAt(4) === "\n";
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text.charAt(4) === "";
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text.startsWith("a2");