useAtIndex

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JavaScript (and super languages)

Summary

Rule available since: v1.9.4
Diagnostic Category: lint/style/useAtIndex
This rule has an unsafe fix.
The default severity of this rule is information.
Sources:
- Inspired from unicorn/prefer-at

How to configure

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

Description

Use at() instead of integer index access.

Accessing an element at the end of an array or a string is inconvenient because you have to subtract the length of the array or the string from the backward 1-based index of the element to access. For example, to access the last element of an array or a string, you would have to write array[array.length - 1]. A more convenient way to achieve the same thing is to use the at() method with a negative index. To access the last element of an array or a string just write array.at(-1).

This rule enforces the usage of at() over index access, charAt(), and slice()[0] when at() is more convenient.

Examples

Invalid

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const foo = array[array.length - 1];

code-block.js:1:13 lint/style/useAtIndex  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Prefer X.at(-Y) over X[X.length - Y].
  
  > 1 │ const foo = array[array.length - 1];
      │             ^^^^^^^^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Using .at() is more convenient and is easier to read.
  
  ℹ Unsafe fix: Use .at().
  
    1   │ - const·foo·=·array[array.length·-·1];
      1 │ + const·foo·=·array.at(-1);
    2 2 │

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const foo = array[array.length - 5];

code-block.js:1:13 lint/style/useAtIndex  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Prefer X.at(-Y) over X[X.length - Y].
  
  > 1 │ const foo = array[array.length - 5];
      │             ^^^^^^^^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Using .at() is more convenient and is easier to read.
  
  ℹ Unsafe fix: Use .at().
  
    1   │ - const·foo·=·array[array.length·-·5];
      1 │ + const·foo·=·array.at(-5);
    2 2 │

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const foo = array.slice(-1)[0];

code-block.js:1:13 lint/style/useAtIndex  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Prefer X.at(Y) over X.slice(Y)[0].
  
  > 1 │ const foo = array.slice(-1)[0];
      │             ^^^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Using .at() is more convenient and is easier to read.
  
  ℹ Unsafe fix: Use .at().
  
    1   │ - const·foo·=·array.slice(-1)[0];
      1 │ + const·foo·=·array.at(-1);
    2 2 │

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const foo = array.slice(-1).pop();

code-block.js:1:13 lint/style/useAtIndex  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Prefer X.at(-1) over X.slice(-a).pop().
  
  > 1 │ const foo = array.slice(-1).pop();
      │             ^^^^^^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Using .at() is more convenient and is easier to read.
  
  ℹ Unsafe fix: Use .at().
  
    1   │ - const·foo·=·array.slice(-1).pop();
      1 │ + const·foo·=·array.at(-1);
    2 2 │

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const foo = array.slice(-5).shift();

code-block.js:1:13 lint/style/useAtIndex  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Prefer X.at(Y) over X.slice(Y).shift().
  
  > 1 │ const foo = array.slice(-5).shift();
      │             ^^^^^^^^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Using .at() is more convenient and is easier to read.
  
  ℹ Unsafe fix: Use .at().
  
    1   │ - const·foo·=·array.slice(-5).shift();
      1 │ + const·foo·=·array.at(-5);
    2 2 │

1
const foo = string.charAt(string.length - 5);

code-block.js:1:13 lint/style/useAtIndex  FIXABLE  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

  ℹ Prefer X.at(-Y) over X.charAt(X.length - Y).
  
  > 1 │ const foo = string.charAt(string.length - 5);
      │             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    2 │ 
  
  ℹ Using .at() is more convenient and is easier to read.
  
  ℹ Unsafe fix: Use .at().
  
    1   │ - const·foo·=·string.charAt(string.length·-·5);
      1 │ + const·foo·=·string.at(-5);
    2 2 │

Valid

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const foo = array.at(-1);

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const foo = array.at(-5);

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const foo = array[100];

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const foo = array.at(array.length - 1);

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array[array.length - 1] = foo;