Cognitive Complexity

Cognitive Complexity is a measure of how difficult a unit of code is to intuitively understand. Unlike Cyclomatic Complexity, which determines how difficult your code will be to test, Cognitive Complexity tells you how difficult your code will be to read and comprehend.

A method's cognitive complexity is based on a few simple rules:

• Code is not considered more complex when it uses shorthand that the language provides for collapsing multiple statements into one
• Code is considered more complex for each "break in the linear flow of the code"
• Code is considered more complex when "flow breaking structures are nested"

Further reading

• Cognitive Complexity docs
• Cognitive Complexity: A new way of measuring understandability

Cognitive Complexity

Cognitive Complexity is a measure of how difficult a unit of code is to intuitively understand. Unlike Cyclomatic Complexity, which determines how difficult your code will be to test, Cognitive Complexity tells you how difficult your code will be to read and comprehend.

A method's cognitive complexity is based on a few simple rules:

• Code is not considered more complex when it uses shorthand that the language provides for collapsing multiple statements into one
• Code is considered more complex for each "break in the linear flow of the code"
• Code is considered more complex when "flow breaking structures are nested"

Further reading

• Cognitive Complexity docs
• Cognitive Complexity: A new way of measuring understandability

Duplicated Code

Duplicated code can lead to software that is hard to understand and difficult to change. The Don't Repeat Yourself (DRY) principle states:

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

When you violate DRY, bugs and maintenance problems are sure to follow. Duplicated code has a tendency to both continue to replicate and also to diverge (leaving bugs as two similar implementations differ in subtle ways).

Tuning

This issue has a mass of 50.

We set useful threshold defaults for the languages we support but you may want to adjust these settings based on your project guidelines.

The threshold configuration represents the minimum mass a code block must have to be analyzed for duplication. The lower the threshold, the more fine-grained the comparison.

If the engine is too easily reporting duplication, try raising the threshold. If you suspect that the engine isn't catching enough duplication, try lowering the threshold. The best setting tends to differ from language to language.

See codeclimate-duplication's documentation for more information about tuning the mass threshold in your .codeclimate.yml.

Refactorings

• Extract Method
• Extract Class
• Form Template Method
• Introduce Null Object
• Pull Up Method
• Pull Up Field
• Substitute Algorithm

Further Reading

• Don't Repeat Yourself on the C2 Wiki
• Duplicated Code on SourceMaking
• Refactoring: Improving the Design of Existing Code by Martin Fowler. Duplicated Code, p76

Duplicated Code

Duplicated code can lead to software that is hard to understand and difficult to change. The Don't Repeat Yourself (DRY) principle states:

Every piece of knowledge must have a single, unambiguous, authoritative representation within a system.

When you violate DRY, bugs and maintenance problems are sure to follow. Duplicated code has a tendency to both continue to replicate and also to diverge (leaving bugs as two similar implementations differ in subtle ways).

Tuning

This issue has a mass of 50.

We set useful threshold defaults for the languages we support but you may want to adjust these settings based on your project guidelines.

The threshold configuration represents the minimum mass a code block must have to be analyzed for duplication. The lower the threshold, the more fine-grained the comparison.

If the engine is too easily reporting duplication, try raising the threshold. If you suspect that the engine isn't catching enough duplication, try lowering the threshold. The best setting tends to differ from language to language.

See codeclimate-duplication's documentation for more information about tuning the mass threshold in your .codeclimate.yml.

Refactorings

• Extract Method
• Extract Class
• Form Template Method
• Introduce Null Object
• Pull Up Method
• Pull Up Field
• Substitute Algorithm

Further Reading

• Don't Repeat Yourself on the C2 Wiki
• Duplicated Code on SourceMaking
• Refactoring: Improving the Design of Existing Code by Martin Fowler. Duplicated Code, p76

Rule: no-var-keyword

Disallows usage of the var keyword.

Use let or const instead.

Rationale

Declaring variables using var has several edge case behaviors that make var unsuitable for modern code. Variables declared by var have their parent function block as their scope, ignoring other control flow statements. vars have declaration "hoisting" (similar to functions) and can appear to be used before declaration.

Variables declared by const and let instead have as their scope the block in which they are defined, and are not allowed to used before declaration or be re-declared with another const or let.

Notes

• Has Fix

Config

Not configurable.

Examples

"no-var-keyword": true

For more information see this page.

Rule: no-var-keyword

Disallows usage of the var keyword.

Use let or const instead.

Rationale

Declaring variables using var has several edge case behaviors that make var unsuitable for modern code. Variables declared by var have their parent function block as their scope, ignoring other control flow statements. vars have declaration "hoisting" (similar to functions) and can appear to be used before declaration.

Variables declared by const and let instead have as their scope the block in which they are defined, and are not allowed to used before declaration or be re-declared with another const or let.

Notes

• Has Fix

Config

Not configurable.

Examples

"no-var-keyword": true

For more information see this page.

Rule: no-var-keyword

Disallows usage of the var keyword.

Use let or const instead.

Rationale

Declaring variables using var has several edge case behaviors that make var unsuitable for modern code. Variables declared by var have their parent function block as their scope, ignoring other control flow statements. vars have declaration "hoisting" (similar to functions) and can appear to be used before declaration.

Variables declared by const and let instead have as their scope the block in which they are defined, and are not allowed to used before declaration or be re-declared with another const or let.

Notes

• Has Fix

Config

Not configurable.

Examples

"no-var-keyword": true

For more information see this page.

Rule: no-var-keyword

Disallows usage of the var keyword.

Use let or const instead.

Rationale

Declaring variables using var has several edge case behaviors that make var unsuitable for modern code. Variables declared by var have their parent function block as their scope, ignoring other control flow statements. vars have declaration "hoisting" (similar to functions) and can appear to be used before declaration.

Variables declared by const and let instead have as their scope the block in which they are defined, and are not allowed to used before declaration or be re-declared with another const or let.

Notes

• Has Fix

Config

Not configurable.

Examples

"no-var-keyword": true

For more information see this page.

Rule: no-var-keyword

Disallows usage of the var keyword.

Use let or const instead.

Rationale

Declaring variables using var has several edge case behaviors that make var unsuitable for modern code. Variables declared by var have their parent function block as their scope, ignoring other control flow statements. vars have declaration "hoisting" (similar to functions) and can appear to be used before declaration.

Variables declared by const and let instead have as their scope the block in which they are defined, and are not allowed to used before declaration or be re-declared with another const or let.

Notes

• Has Fix

Config

Not configurable.

Examples

"no-var-keyword": true

For more information see this page.

Rule: only-arrow-functions

Disallows traditional (non-arrow) function expressions.

Note that non-arrow functions are allowed if 'this' appears somewhere in its body (as such functions cannot be converted to arrow functions).

Rationale

Traditional functions don't bind lexical scope, which can lead to unexpected behavior when accessing 'this'.

Config

Two arguments may be optionally provided:

• "allow-declarations" allows standalone function declarations.
• "allow-named-functions" allows the expression function foo() {} but not function() {}.

Examples

"only-arrow-functions": true

"only-arrow-functions": true,allow-declarations,allow-named-functions

Schema

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "allow-declarations",
      "allow-named-functions"
    ]
  },
  "minLength": 0,
  "maxLength": 1
}

For more information see this page.

Rule: prefer-const

Requires that variable declarations use const instead of let and var if possible.

If a variable is only assigned to once when it is declared, it should be declared using 'const'

Notes

• Has Fix

Config

An optional object containing the property "destructuring" with two possible values:

• "any" (default) - If any variable in destructuring can be const, this rule warns for those variables.
• "all" - Only warns if all variables in destructuring can be const.

Examples

"prefer-const": true

"prefer-const": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "destructuring": {
      "type": "string",
      "enum": [
        "all",
        "any"
      ]
    }
  }
}

For more information see this page.

Rule: prefer-const

Requires that variable declarations use const instead of let and var if possible.

If a variable is only assigned to once when it is declared, it should be declared using 'const'

Notes

• Has Fix

Config

An optional object containing the property "destructuring" with two possible values:

• "any" (default) - If any variable in destructuring can be const, this rule warns for those variables.
• "all" - Only warns if all variables in destructuring can be const.

Examples

"prefer-const": true

"prefer-const": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "destructuring": {
      "type": "string",
      "enum": [
        "all",
        "any"
      ]
    }
  }
}

For more information see this page.

Rule: prefer-object-spread

Enforces the use of the ES2018 object spread operator over Object.assign() where appropriate.

Rationale

Object spread allows for better type checking and inference.

Notes

• Has Fix

Config

Not configurable.

Examples

"prefer-object-spread": true

For more information see this page.

Rule: prefer-const

Requires that variable declarations use const instead of let and var if possible.

If a variable is only assigned to once when it is declared, it should be declared using 'const'

Notes

• Has Fix

Config

An optional object containing the property "destructuring" with two possible values:

• "any" (default) - If any variable in destructuring can be const, this rule warns for those variables.
• "all" - Only warns if all variables in destructuring can be const.

Examples

"prefer-const": true

"prefer-const": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "destructuring": {
      "type": "string",
      "enum": [
        "all",
        "any"
      ]
    }
  }
}

For more information see this page.

Rule: no-var-keyword

Disallows usage of the var keyword.

Use let or const instead.

Rationale

Declaring variables using var has several edge case behaviors that make var unsuitable for modern code. Variables declared by var have their parent function block as their scope, ignoring other control flow statements. vars have declaration "hoisting" (similar to functions) and can appear to be used before declaration.

Variables declared by const and let instead have as their scope the block in which they are defined, and are not allowed to used before declaration or be re-declared with another const or let.

Notes

• Has Fix

Config

Not configurable.

Examples

"no-var-keyword": true

For more information see this page.

Rule: only-arrow-functions

Disallows traditional (non-arrow) function expressions.

Note that non-arrow functions are allowed if 'this' appears somewhere in its body (as such functions cannot be converted to arrow functions).

Rationale

Traditional functions don't bind lexical scope, which can lead to unexpected behavior when accessing 'this'.

Config

Two arguments may be optionally provided:

• "allow-declarations" allows standalone function declarations.
• "allow-named-functions" allows the expression function foo() {} but not function() {}.

Examples

"only-arrow-functions": true

"only-arrow-functions": true,allow-declarations,allow-named-functions

Schema

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "allow-declarations",
      "allow-named-functions"
    ]
  },
  "minLength": 0,
  "maxLength": 1
}

For more information see this page.

Rule: prefer-const

Requires that variable declarations use const instead of let and var if possible.

If a variable is only assigned to once when it is declared, it should be declared using 'const'

Notes

• Has Fix

Config

An optional object containing the property "destructuring" with two possible values:

• "any" (default) - If any variable in destructuring can be const, this rule warns for those variables.
• "all" - Only warns if all variables in destructuring can be const.

Examples

"prefer-const": true

"prefer-const": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "destructuring": {
      "type": "string",
      "enum": [
        "all",
        "any"
      ]
    }
  }
}

For more information see this page.

Rule: no-var-keyword

Disallows usage of the var keyword.

Use let or const instead.

Rationale

Declaring variables using var has several edge case behaviors that make var unsuitable for modern code. Variables declared by var have their parent function block as their scope, ignoring other control flow statements. vars have declaration "hoisting" (similar to functions) and can appear to be used before declaration.

Variables declared by const and let instead have as their scope the block in which they are defined, and are not allowed to used before declaration or be re-declared with another const or let.

Notes

• Has Fix

Config

Not configurable.

Examples

"no-var-keyword": true

For more information see this page.

Rule: object-literal-shorthand

Enforces/disallows use of ES6 object literal shorthand.

Notes

• Has Fix

Config

"always" assumed to be default option, thus with no options provided the rule enforces object literal methods and properties shorthands. With "never" option provided, any shorthand object literal syntax causes an error.

The rule can be configured in a more granular way. With {"property": "never"} provided (which is equivalent to {"property": "never", "method": "always"}), the rule only flags property shorthand assignments, and respectively with {"method": "never"} (equivalent to {"property": "always", "method": "never"}), the rule fails only on method shorthands.

Examples

"object-literal-shorthand": true

"object-literal-shorthand": true,never

"object-literal-shorthand": true,[object Object]

Schema

{
  "oneOf": [
    {
      "type": "string",
      "enum": [
        "never"
      ]
    },
    {
      "type": "object",
      "properties": {
        "property": {
          "type": "string",
          "enum": [
            "never"
          ]
        },
        "method": {
          "type": "string",
          "enum": [
            "never"
          ]
        }
      },
      "minProperties": 1,
      "maxProperties": 2
    }
  ]
}

For more information see this page.

Rule: only-arrow-functions

Disallows traditional (non-arrow) function expressions.

Note that non-arrow functions are allowed if 'this' appears somewhere in its body (as such functions cannot be converted to arrow functions).

Rationale

Traditional functions don't bind lexical scope, which can lead to unexpected behavior when accessing 'this'.

Config

Two arguments may be optionally provided:

• "allow-declarations" allows standalone function declarations.
• "allow-named-functions" allows the expression function foo() {} but not function() {}.

Examples

"only-arrow-functions": true

"only-arrow-functions": true,allow-declarations,allow-named-functions

Schema

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "allow-declarations",
      "allow-named-functions"
    ]
  },
  "minLength": 0,
  "maxLength": 1
}

For more information see this page.

Rule: prefer-const

Requires that variable declarations use const instead of let and var if possible.

If a variable is only assigned to once when it is declared, it should be declared using 'const'

Notes

• Has Fix

Config

An optional object containing the property "destructuring" with two possible values:

• "any" (default) - If any variable in destructuring can be const, this rule warns for those variables.
• "all" - Only warns if all variables in destructuring can be const.

Examples

"prefer-const": true

"prefer-const": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "destructuring": {
      "type": "string",
      "enum": [
        "all",
        "any"
      ]
    }
  }
}

For more information see this page.

Rule: no-var-keyword

Disallows usage of the var keyword.

Use let or const instead.

Rationale

Declaring variables using var has several edge case behaviors that make var unsuitable for modern code. Variables declared by var have their parent function block as their scope, ignoring other control flow statements. vars have declaration "hoisting" (similar to functions) and can appear to be used before declaration.

Variables declared by const and let instead have as their scope the block in which they are defined, and are not allowed to used before declaration or be re-declared with another const or let.

Notes

• Has Fix

Config

Not configurable.

Examples

"no-var-keyword": true

For more information see this page.

Rule: prefer-const

Requires that variable declarations use const instead of let and var if possible.

If a variable is only assigned to once when it is declared, it should be declared using 'const'

Notes

• Has Fix

Config

An optional object containing the property "destructuring" with two possible values:

• "any" (default) - If any variable in destructuring can be const, this rule warns for those variables.
• "all" - Only warns if all variables in destructuring can be const.

Examples

"prefer-const": true

"prefer-const": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "destructuring": {
      "type": "string",
      "enum": [
        "all",
        "any"
      ]
    }
  }
}

For more information see this page.

Rule: prefer-const

Requires that variable declarations use const instead of let and var if possible.

If a variable is only assigned to once when it is declared, it should be declared using 'const'

Notes

• Has Fix

Config

An optional object containing the property "destructuring" with two possible values:

• "any" (default) - If any variable in destructuring can be const, this rule warns for those variables.
• "all" - Only warns if all variables in destructuring can be const.

Examples

"prefer-const": true

"prefer-const": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "destructuring": {
      "type": "string",
      "enum": [
        "all",
        "any"
      ]
    }
  }
}

For more information see this page.

Rule: no-var-keyword

Disallows usage of the var keyword.

Use let or const instead.

Rationale

Declaring variables using var has several edge case behaviors that make var unsuitable for modern code. Variables declared by var have their parent function block as their scope, ignoring other control flow statements. vars have declaration "hoisting" (similar to functions) and can appear to be used before declaration.

Variables declared by const and let instead have as their scope the block in which they are defined, and are not allowed to used before declaration or be re-declared with another const or let.

Notes

• Has Fix

Config

Not configurable.

Examples

"no-var-keyword": true

For more information see this page.

Rule: only-arrow-functions

Disallows traditional (non-arrow) function expressions.

Note that non-arrow functions are allowed if 'this' appears somewhere in its body (as such functions cannot be converted to arrow functions).

Rationale

Traditional functions don't bind lexical scope, which can lead to unexpected behavior when accessing 'this'.

Config

Two arguments may be optionally provided:

• "allow-declarations" allows standalone function declarations.
• "allow-named-functions" allows the expression function foo() {} but not function() {}.

Examples

"only-arrow-functions": true

"only-arrow-functions": true,allow-declarations,allow-named-functions

Schema

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "allow-declarations",
      "allow-named-functions"
    ]
  },
  "minLength": 0,
  "maxLength": 1
}

For more information see this page.

Rule: prefer-const

Requires that variable declarations use const instead of let and var if possible.

If a variable is only assigned to once when it is declared, it should be declared using 'const'

Notes

• Has Fix

Config

An optional object containing the property "destructuring" with two possible values:

• "any" (default) - If any variable in destructuring can be const, this rule warns for those variables.
• "all" - Only warns if all variables in destructuring can be const.

Examples

"prefer-const": true

"prefer-const": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "destructuring": {
      "type": "string",
      "enum": [
        "all",
        "any"
      ]
    }
  }
}

For more information see this page.

Rule: prefer-const

Requires that variable declarations use const instead of let and var if possible.

If a variable is only assigned to once when it is declared, it should be declared using 'const'

Notes

• Has Fix

Config

An optional object containing the property "destructuring" with two possible values:

• "any" (default) - If any variable in destructuring can be const, this rule warns for those variables.
• "all" - Only warns if all variables in destructuring can be const.

Examples

"prefer-const": true

"prefer-const": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "destructuring": {
      "type": "string",
      "enum": [
        "all",
        "any"
      ]
    }
  }
}

For more information see this page.

Rule: no-object-literal-type-assertion

Forbids an object literal to appear in a type assertion expression. Casting to any or to unknown is still allowed.

Rationale

Always prefer const x: T = { ... }; to const x = { ... } as T;. The type assertion in the latter case is either unnecessary or hides an error. The compiler will warn for excess properties with this syntax, but not missing required fields. For example: const x: { foo: number } = {} will fail to compile, but const x = {} as { foo: number } will succeed. Additionally, the const assertion const x = { foo: 1 } as const, introduced in TypeScript 3.4, is considered beneficial and is ignored by this rule.

Notes

• TypeScript Only

Config

One option may be configured:

• allow-arguments allows type assertions to be used on object literals inside call expressions.

Examples

"no-object-literal-type-assertion": true

"no-object-literal-type-assertion": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "allow-arguments": {
      "type": "boolean"
    }
  },
  "additionalProperties": false
}

For more information see this page.

Rule: no-var-keyword

Disallows usage of the var keyword.

Use let or const instead.

Rationale

Declaring variables using var has several edge case behaviors that make var unsuitable for modern code. Variables declared by var have their parent function block as their scope, ignoring other control flow statements. vars have declaration "hoisting" (similar to functions) and can appear to be used before declaration.

Variables declared by const and let instead have as their scope the block in which they are defined, and are not allowed to used before declaration or be re-declared with another const or let.

Notes

• Has Fix

Config

Not configurable.

Examples

"no-var-keyword": true

For more information see this page.

Rule: only-arrow-functions

Disallows traditional (non-arrow) function expressions.

Note that non-arrow functions are allowed if 'this' appears somewhere in its body (as such functions cannot be converted to arrow functions).

Rationale

Traditional functions don't bind lexical scope, which can lead to unexpected behavior when accessing 'this'.

Config

Two arguments may be optionally provided:

• "allow-declarations" allows standalone function declarations.
• "allow-named-functions" allows the expression function foo() {} but not function() {}.

Examples

"only-arrow-functions": true

"only-arrow-functions": true,allow-declarations,allow-named-functions

Schema

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "allow-declarations",
      "allow-named-functions"
    ]
  },
  "minLength": 0,
  "maxLength": 1
}

For more information see this page.

Rule: prefer-const

Requires that variable declarations use const instead of let and var if possible.

If a variable is only assigned to once when it is declared, it should be declared using 'const'

Notes

• Has Fix

Config

An optional object containing the property "destructuring" with two possible values:

• "any" (default) - If any variable in destructuring can be const, this rule warns for those variables.
• "all" - Only warns if all variables in destructuring can be const.

Examples

"prefer-const": true

"prefer-const": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "destructuring": {
      "type": "string",
      "enum": [
        "all",
        "any"
      ]
    }
  }
}

For more information see this page.

Rule: prefer-const

Requires that variable declarations use const instead of let and var if possible.

If a variable is only assigned to once when it is declared, it should be declared using 'const'

Notes

• Has Fix

Config

An optional object containing the property "destructuring" with two possible values:

• "any" (default) - If any variable in destructuring can be const, this rule warns for those variables.
• "all" - Only warns if all variables in destructuring can be const.

Examples

"prefer-const": true

"prefer-const": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "destructuring": {
      "type": "string",
      "enum": [
        "all",
        "any"
      ]
    }
  }
}

For more information see this page.

Rule: prefer-const

Requires that variable declarations use const instead of let and var if possible.

If a variable is only assigned to once when it is declared, it should be declared using 'const'

Notes

• Has Fix

Config

An optional object containing the property "destructuring" with two possible values:

• "any" (default) - If any variable in destructuring can be const, this rule warns for those variables.
• "all" - Only warns if all variables in destructuring can be const.

Examples

"prefer-const": true

"prefer-const": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "destructuring": {
      "type": "string",
      "enum": [
        "all",
        "any"
      ]
    }
  }
}

For more information see this page.

Rule: prefer-const

Requires that variable declarations use const instead of let and var if possible.

If a variable is only assigned to once when it is declared, it should be declared using 'const'

Notes

• Has Fix

Config

An optional object containing the property "destructuring" with two possible values:

• "any" (default) - If any variable in destructuring can be const, this rule warns for those variables.
• "all" - Only warns if all variables in destructuring can be const.

Examples

"prefer-const": true

"prefer-const": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "destructuring": {
      "type": "string",
      "enum": [
        "all",
        "any"
      ]
    }
  }
}

For more information see this page.

Rule: prefer-const

Requires that variable declarations use const instead of let and var if possible.

If a variable is only assigned to once when it is declared, it should be declared using 'const'

Notes

• Has Fix

Config

An optional object containing the property "destructuring" with two possible values:

• "any" (default) - If any variable in destructuring can be const, this rule warns for those variables.
• "all" - Only warns if all variables in destructuring can be const.

Examples

"prefer-const": true

"prefer-const": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "destructuring": {
      "type": "string",
      "enum": [
        "all",
        "any"
      ]
    }
  }
}

For more information see this page.

Rule: no-var-keyword

Disallows usage of the var keyword.

Use let or const instead.

Rationale

Declaring variables using var has several edge case behaviors that make var unsuitable for modern code. Variables declared by var have their parent function block as their scope, ignoring other control flow statements. vars have declaration "hoisting" (similar to functions) and can appear to be used before declaration.

Variables declared by const and let instead have as their scope the block in which they are defined, and are not allowed to used before declaration or be re-declared with another const or let.

Notes

• Has Fix

Config

Not configurable.

Examples

"no-var-keyword": true

For more information see this page.

Rule: no-var-keyword

Disallows usage of the var keyword.

Use let or const instead.

Rationale

Declaring variables using var has several edge case behaviors that make var unsuitable for modern code. Variables declared by var have their parent function block as their scope, ignoring other control flow statements. vars have declaration "hoisting" (similar to functions) and can appear to be used before declaration.

Variables declared by const and let instead have as their scope the block in which they are defined, and are not allowed to used before declaration or be re-declared with another const or let.

Notes

• Has Fix

Config

Not configurable.

Examples

"no-var-keyword": true

For more information see this page.

Rule: prefer-const

Requires that variable declarations use const instead of let and var if possible.

If a variable is only assigned to once when it is declared, it should be declared using 'const'

Notes

• Has Fix

Config

An optional object containing the property "destructuring" with two possible values:

• "any" (default) - If any variable in destructuring can be const, this rule warns for those variables.
• "all" - Only warns if all variables in destructuring can be const.

Examples

"prefer-const": true

"prefer-const": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "destructuring": {
      "type": "string",
      "enum": [
        "all",
        "any"
      ]
    }
  }
}

For more information see this page.

Rule: no-var-keyword

Disallows usage of the var keyword.

Use let or const instead.

Rationale

Declaring variables using var has several edge case behaviors that make var unsuitable for modern code. Variables declared by var have their parent function block as their scope, ignoring other control flow statements. vars have declaration "hoisting" (similar to functions) and can appear to be used before declaration.

Variables declared by const and let instead have as their scope the block in which they are defined, and are not allowed to used before declaration or be re-declared with another const or let.

Notes

• Has Fix

Config

Not configurable.

Examples

"no-var-keyword": true

For more information see this page.

Rule: no-object-literal-type-assertion

Forbids an object literal to appear in a type assertion expression. Casting to any or to unknown is still allowed.

Rationale

Always prefer const x: T = { ... }; to const x = { ... } as T;. The type assertion in the latter case is either unnecessary or hides an error. The compiler will warn for excess properties with this syntax, but not missing required fields. For example: const x: { foo: number } = {} will fail to compile, but const x = {} as { foo: number } will succeed. Additionally, the const assertion const x = { foo: 1 } as const, introduced in TypeScript 3.4, is considered beneficial and is ignored by this rule.

Notes

• TypeScript Only

Config

One option may be configured:

• allow-arguments allows type assertions to be used on object literals inside call expressions.

Examples

"no-object-literal-type-assertion": true

"no-object-literal-type-assertion": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "allow-arguments": {
      "type": "boolean"
    }
  },
  "additionalProperties": false
}

For more information see this page.

Rule: no-var-keyword

Disallows usage of the var keyword.

Use let or const instead.

Rationale

Declaring variables using var has several edge case behaviors that make var unsuitable for modern code. Variables declared by var have their parent function block as their scope, ignoring other control flow statements. vars have declaration "hoisting" (similar to functions) and can appear to be used before declaration.

Variables declared by const and let instead have as their scope the block in which they are defined, and are not allowed to used before declaration or be re-declared with another const or let.

Notes

• Has Fix

Config

Not configurable.

Examples

"no-var-keyword": true

For more information see this page.

Rule: no-var-keyword

Disallows usage of the var keyword.

Use let or const instead.

Rationale

Declaring variables using var has several edge case behaviors that make var unsuitable for modern code. Variables declared by var have their parent function block as their scope, ignoring other control flow statements. vars have declaration "hoisting" (similar to functions) and can appear to be used before declaration.

Variables declared by const and let instead have as their scope the block in which they are defined, and are not allowed to used before declaration or be re-declared with another const or let.

Notes

• Has Fix

Config

Not configurable.

Examples

"no-var-keyword": true

For more information see this page.

Rule: no-var-keyword

Disallows usage of the var keyword.

Use let or const instead.

Rationale

Declaring variables using var has several edge case behaviors that make var unsuitable for modern code. Variables declared by var have their parent function block as their scope, ignoring other control flow statements. vars have declaration "hoisting" (similar to functions) and can appear to be used before declaration.

Variables declared by const and let instead have as their scope the block in which they are defined, and are not allowed to used before declaration or be re-declared with another const or let.

Notes

• Has Fix

Config

Not configurable.

Examples

"no-var-keyword": true

For more information see this page.

Rule: no-var-keyword

Disallows usage of the var keyword.

Use let or const instead.

Rationale

Declaring variables using var has several edge case behaviors that make var unsuitable for modern code. Variables declared by var have their parent function block as their scope, ignoring other control flow statements. vars have declaration "hoisting" (similar to functions) and can appear to be used before declaration.

Variables declared by const and let instead have as their scope the block in which they are defined, and are not allowed to used before declaration or be re-declared with another const or let.

Notes

• Has Fix

Config

Not configurable.

Examples

"no-var-keyword": true

For more information see this page.

Rule: only-arrow-functions

Disallows traditional (non-arrow) function expressions.

Note that non-arrow functions are allowed if 'this' appears somewhere in its body (as such functions cannot be converted to arrow functions).

Rationale

Traditional functions don't bind lexical scope, which can lead to unexpected behavior when accessing 'this'.

Config

Two arguments may be optionally provided:

• "allow-declarations" allows standalone function declarations.
• "allow-named-functions" allows the expression function foo() {} but not function() {}.

Examples

"only-arrow-functions": true

"only-arrow-functions": true,allow-declarations,allow-named-functions

Schema

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "allow-declarations",
      "allow-named-functions"
    ]
  },
  "minLength": 0,
  "maxLength": 1
}

For more information see this page.

Rule: only-arrow-functions

Disallows traditional (non-arrow) function expressions.

Note that non-arrow functions are allowed if 'this' appears somewhere in its body (as such functions cannot be converted to arrow functions).

Rationale

Traditional functions don't bind lexical scope, which can lead to unexpected behavior when accessing 'this'.

Config

Two arguments may be optionally provided:

• "allow-declarations" allows standalone function declarations.
• "allow-named-functions" allows the expression function foo() {} but not function() {}.

Examples

"only-arrow-functions": true

"only-arrow-functions": true,allow-declarations,allow-named-functions

Schema

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "allow-declarations",
      "allow-named-functions"
    ]
  },
  "minLength": 0,
  "maxLength": 1
}

For more information see this page.

Rule: no-object-literal-type-assertion

Forbids an object literal to appear in a type assertion expression. Casting to any or to unknown is still allowed.

Rationale

Always prefer const x: T = { ... }; to const x = { ... } as T;. The type assertion in the latter case is either unnecessary or hides an error. The compiler will warn for excess properties with this syntax, but not missing required fields. For example: const x: { foo: number } = {} will fail to compile, but const x = {} as { foo: number } will succeed. Additionally, the const assertion const x = { foo: 1 } as const, introduced in TypeScript 3.4, is considered beneficial and is ignored by this rule.

Notes

• TypeScript Only

Config

One option may be configured:

• allow-arguments allows type assertions to be used on object literals inside call expressions.

Examples

"no-object-literal-type-assertion": true

"no-object-literal-type-assertion": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "allow-arguments": {
      "type": "boolean"
    }
  },
  "additionalProperties": false
}

For more information see this page.

Rule: no-var-keyword

Disallows usage of the var keyword.

Use let or const instead.

Rationale

Declaring variables using var has several edge case behaviors that make var unsuitable for modern code. Variables declared by var have their parent function block as their scope, ignoring other control flow statements. vars have declaration "hoisting" (similar to functions) and can appear to be used before declaration.

Variables declared by const and let instead have as their scope the block in which they are defined, and are not allowed to used before declaration or be re-declared with another const or let.

Notes

• Has Fix

Config

Not configurable.

Examples

"no-var-keyword": true

For more information see this page.

Rule: no-var-keyword

Disallows usage of the var keyword.

Use let or const instead.

Rationale

Declaring variables using var has several edge case behaviors that make var unsuitable for modern code. Variables declared by var have their parent function block as their scope, ignoring other control flow statements. vars have declaration "hoisting" (similar to functions) and can appear to be used before declaration.

Variables declared by const and let instead have as their scope the block in which they are defined, and are not allowed to used before declaration or be re-declared with another const or let.

Notes

• Has Fix

Config

Not configurable.

Examples

"no-var-keyword": true

For more information see this page.

Rule: prefer-const

Requires that variable declarations use const instead of let and var if possible.

If a variable is only assigned to once when it is declared, it should be declared using 'const'

Notes

• Has Fix

Config

An optional object containing the property "destructuring" with two possible values:

• "any" (default) - If any variable in destructuring can be const, this rule warns for those variables.
• "all" - Only warns if all variables in destructuring can be const.

Examples

"prefer-const": true

"prefer-const": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "destructuring": {
      "type": "string",
      "enum": [
        "all",
        "any"
      ]
    }
  }
}

For more information see this page.

Rule: object-literal-shorthand

Enforces/disallows use of ES6 object literal shorthand.

Notes

• Has Fix

Config

"always" assumed to be default option, thus with no options provided the rule enforces object literal methods and properties shorthands. With "never" option provided, any shorthand object literal syntax causes an error.

The rule can be configured in a more granular way. With {"property": "never"} provided (which is equivalent to {"property": "never", "method": "always"}), the rule only flags property shorthand assignments, and respectively with {"method": "never"} (equivalent to {"property": "always", "method": "never"}), the rule fails only on method shorthands.

Examples

"object-literal-shorthand": true

"object-literal-shorthand": true,never

"object-literal-shorthand": true,[object Object]

Schema

{
  "oneOf": [
    {
      "type": "string",
      "enum": [
        "never"
      ]
    },
    {
      "type": "object",
      "properties": {
        "property": {
          "type": "string",
          "enum": [
            "never"
          ]
        },
        "method": {
          "type": "string",
          "enum": [
            "never"
          ]
        }
      },
      "minProperties": 1,
      "maxProperties": 2
    }
  ]
}

For more information see this page.

Rule: only-arrow-functions

Disallows traditional (non-arrow) function expressions.

Note that non-arrow functions are allowed if 'this' appears somewhere in its body (as such functions cannot be converted to arrow functions).

Rationale

Traditional functions don't bind lexical scope, which can lead to unexpected behavior when accessing 'this'.

Config

Two arguments may be optionally provided:

• "allow-declarations" allows standalone function declarations.
• "allow-named-functions" allows the expression function foo() {} but not function() {}.

Examples

"only-arrow-functions": true

"only-arrow-functions": true,allow-declarations,allow-named-functions

Schema

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "allow-declarations",
      "allow-named-functions"
    ]
  },
  "minLength": 0,
  "maxLength": 1
}

For more information see this page.

Rule: prefer-const

Requires that variable declarations use const instead of let and var if possible.

If a variable is only assigned to once when it is declared, it should be declared using 'const'

Notes

• Has Fix

Config

An optional object containing the property "destructuring" with two possible values:

• "any" (default) - If any variable in destructuring can be const, this rule warns for those variables.
• "all" - Only warns if all variables in destructuring can be const.

Examples

"prefer-const": true

"prefer-const": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "destructuring": {
      "type": "string",
      "enum": [
        "all",
        "any"
      ]
    }
  }
}

For more information see this page.

Rule: prefer-for-of

Recommends a 'for-of' loop over a standard 'for' loop if the index is only used to access the array being iterated.

Rationale

A for(... of ...) loop is easier to implement and read when the index is not needed.

Config

Not configurable.

Examples

"prefer-for-of": true

For more information see this page.

Rule: no-var-keyword

Disallows usage of the var keyword.

Use let or const instead.

Rationale

Declaring variables using var has several edge case behaviors that make var unsuitable for modern code. Variables declared by var have their parent function block as their scope, ignoring other control flow statements. vars have declaration "hoisting" (similar to functions) and can appear to be used before declaration.

Variables declared by const and let instead have as their scope the block in which they are defined, and are not allowed to used before declaration or be re-declared with another const or let.

Notes

• Has Fix

Config

Not configurable.

Examples

"no-var-keyword": true

For more information see this page.

Rule: no-var-keyword

Disallows usage of the var keyword.

Use let or const instead.

Rationale

Declaring variables using var has several edge case behaviors that make var unsuitable for modern code. Variables declared by var have their parent function block as their scope, ignoring other control flow statements. vars have declaration "hoisting" (similar to functions) and can appear to be used before declaration.

Variables declared by const and let instead have as their scope the block in which they are defined, and are not allowed to used before declaration or be re-declared with another const or let.

Notes

• Has Fix

Config

Not configurable.

Examples

"no-var-keyword": true

For more information see this page.

Rule: no-var-keyword

Disallows usage of the var keyword.

Use let or const instead.

Rationale

Declaring variables using var has several edge case behaviors that make var unsuitable for modern code. Variables declared by var have their parent function block as their scope, ignoring other control flow statements. vars have declaration "hoisting" (similar to functions) and can appear to be used before declaration.

Variables declared by const and let instead have as their scope the block in which they are defined, and are not allowed to used before declaration or be re-declared with another const or let.

Notes

• Has Fix

Config

Not configurable.

Examples

"no-var-keyword": true

For more information see this page.

Rule: object-literal-shorthand

Enforces/disallows use of ES6 object literal shorthand.

Notes

• Has Fix

Config

"always" assumed to be default option, thus with no options provided the rule enforces object literal methods and properties shorthands. With "never" option provided, any shorthand object literal syntax causes an error.

The rule can be configured in a more granular way. With {"property": "never"} provided (which is equivalent to {"property": "never", "method": "always"}), the rule only flags property shorthand assignments, and respectively with {"method": "never"} (equivalent to {"property": "always", "method": "never"}), the rule fails only on method shorthands.

Examples

"object-literal-shorthand": true

"object-literal-shorthand": true,never

"object-literal-shorthand": true,[object Object]

Schema

{
  "oneOf": [
    {
      "type": "string",
      "enum": [
        "never"
      ]
    },
    {
      "type": "object",
      "properties": {
        "property": {
          "type": "string",
          "enum": [
            "never"
          ]
        },
        "method": {
          "type": "string",
          "enum": [
            "never"
          ]
        }
      },
      "minProperties": 1,
      "maxProperties": 2
    }
  ]
}

For more information see this page.

Rule: prefer-const

Requires that variable declarations use const instead of let and var if possible.

If a variable is only assigned to once when it is declared, it should be declared using 'const'

Notes

• Has Fix

Config

An optional object containing the property "destructuring" with two possible values:

• "any" (default) - If any variable in destructuring can be const, this rule warns for those variables.
• "all" - Only warns if all variables in destructuring can be const.

Examples

"prefer-const": true

"prefer-const": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "destructuring": {
      "type": "string",
      "enum": [
        "all",
        "any"
      ]
    }
  }
}

For more information see this page.

Rule: prefer-const

Requires that variable declarations use const instead of let and var if possible.

If a variable is only assigned to once when it is declared, it should be declared using 'const'

Notes

• Has Fix

Config

An optional object containing the property "destructuring" with two possible values:

• "any" (default) - If any variable in destructuring can be const, this rule warns for those variables.
• "all" - Only warns if all variables in destructuring can be const.

Examples

"prefer-const": true

"prefer-const": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "destructuring": {
      "type": "string",
      "enum": [
        "all",
        "any"
      ]
    }
  }
}

For more information see this page.

Rule: prefer-for-of

Recommends a 'for-of' loop over a standard 'for' loop if the index is only used to access the array being iterated.

Rationale

A for(... of ...) loop is easier to implement and read when the index is not needed.

Config

Not configurable.

Examples

"prefer-for-of": true

For more information see this page.

Rule: no-object-literal-type-assertion

Forbids an object literal to appear in a type assertion expression. Casting to any or to unknown is still allowed.

Rationale

Always prefer const x: T = { ... }; to const x = { ... } as T;. The type assertion in the latter case is either unnecessary or hides an error. The compiler will warn for excess properties with this syntax, but not missing required fields. For example: const x: { foo: number } = {} will fail to compile, but const x = {} as { foo: number } will succeed. Additionally, the const assertion const x = { foo: 1 } as const, introduced in TypeScript 3.4, is considered beneficial and is ignored by this rule.

Notes

• TypeScript Only

Config

One option may be configured:

• allow-arguments allows type assertions to be used on object literals inside call expressions.

Examples

"no-object-literal-type-assertion": true

"no-object-literal-type-assertion": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "allow-arguments": {
      "type": "boolean"
    }
  },
  "additionalProperties": false
}

For more information see this page.