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

Rule: no-unused-expression

Disallows unused expression statements.

Unused expressions are expression statements which are not assignments or function calls (and thus usually no-ops).

Rationale

Detects potential errors where an assignment or function call was intended.

Config

Three arguments may be optionally provided:

• allow-fast-null-checks allows to use logical operators to perform fast null checks and perform method or function calls for side effects (e.g. e && e.preventDefault()).
• allow-new allows 'new' expressions for side effects (e.g. new ModifyGlobalState();.
• allow-tagged-template allows tagged templates for side effects (e.g. this.add\foo`;`.

Examples

"no-unused-expression": true

"no-unused-expression": true,allow-fast-null-checks

Schema

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "allow-fast-null-checks",
      "allow-new",
      "allow-tagged-template"
    ]
  },
  "minLength": 0,
  "maxLength": 3
}

For more information see this page.

Rule: quotemark

Enforces quote character for string literals.

Notes

• Has Fix

Config

Five arguments may be optionally provided:

• "single" enforces single quotes.
• "double" enforces double quotes.
• "backtick" enforces backticks.
• "jsx-single" enforces single quotes for JSX attributes.
• "jsx-double" enforces double quotes for JSX attributes.
• "avoid-template" forbids single-line untagged template strings that do not contain string interpolations. Note that backticks may still be used if "avoid-escape" is enabled and both single and double quotes are present in the string (the latter option takes precedence).
• "avoid-escape" allows you to use the "other" quotemark in cases where escaping would normally be required. For example, [true, "double", "avoid-escape"] would not report a failure on the string literal 'Hello "World"'.

Examples

"quotemark": true,single,avoid-escape,avoid-template

"quotemark": true,single,jsx-double

Schema

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "single",
      "double",
      "backtick",
      "jsx-single",
      "jsx-double",
      "avoid-escape",
      "avoid-template"
    ]
  },
  "minLength": 0,
  "maxLength": 5
}

For more information see this page.

Rule: quotemark

Enforces quote character for string literals.

Notes

• Has Fix

Config

Five arguments may be optionally provided:

• "single" enforces single quotes.
• "double" enforces double quotes.
• "backtick" enforces backticks.
• "jsx-single" enforces single quotes for JSX attributes.
• "jsx-double" enforces double quotes for JSX attributes.
• "avoid-template" forbids single-line untagged template strings that do not contain string interpolations. Note that backticks may still be used if "avoid-escape" is enabled and both single and double quotes are present in the string (the latter option takes precedence).
• "avoid-escape" allows you to use the "other" quotemark in cases where escaping would normally be required. For example, [true, "double", "avoid-escape"] would not report a failure on the string literal 'Hello "World"'.

Examples

"quotemark": true,single,avoid-escape,avoid-template

"quotemark": true,single,jsx-double

Schema

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "single",
      "double",
      "backtick",
      "jsx-single",
      "jsx-double",
      "avoid-escape",
      "avoid-template"
    ]
  },
  "minLength": 0,
  "maxLength": 5
}

For more information see this page.

Rule: semicolon

Enforces consistent semicolon usage at the end of every statement.

Notes

• Has Fix

Config

One of the following arguments must be provided:

• "always" enforces semicolons at the end of every statement.
• "never" disallows semicolons at the end of every statement except for when they are necessary.

The following arguments may be optionally provided:

• "ignore-interfaces" skips checking semicolons at the end of interface members.
• "ignore-bound-class-methods" skips checking semicolons at the end of bound class methods.
• "strict-bound-class-methods" disables any special handling of bound class methods and treats them as any other assignment. This option overrides "ignore-bound-class-methods".

Examples

"semicolon": true,always

"semicolon": true,never

"semicolon": true,always,ignore-interfaces

"semicolon": true,always,ignore-bound-class-methods

Schema

{
  "type": "array",
  "items": [
    {
      "type": "string",
      "enum": [
        "always",
        "never"
      ]
    },
    {
      "type": "string",
      "enum": [
        "ignore-interfaces"
      ]
    }
  ],
  "additionalItems": false
}

For more information see this page.

Rule: semicolon

Enforces consistent semicolon usage at the end of every statement.

Notes

• Has Fix

Config

One of the following arguments must be provided:

• "always" enforces semicolons at the end of every statement.
• "never" disallows semicolons at the end of every statement except for when they are necessary.

The following arguments may be optionally provided:

• "ignore-interfaces" skips checking semicolons at the end of interface members.
• "ignore-bound-class-methods" skips checking semicolons at the end of bound class methods.
• "strict-bound-class-methods" disables any special handling of bound class methods and treats them as any other assignment. This option overrides "ignore-bound-class-methods".

Examples

"semicolon": true,always

"semicolon": true,never

"semicolon": true,always,ignore-interfaces

"semicolon": true,always,ignore-bound-class-methods

Schema

{
  "type": "array",
  "items": [
    {
      "type": "string",
      "enum": [
        "always",
        "never"
      ]
    },
    {
      "type": "string",
      "enum": [
        "ignore-interfaces"
      ]
    }
  ],
  "additionalItems": false
}

For more information see this page.

Rule: quotemark

Enforces quote character for string literals.

Notes

• Has Fix

Config

Five arguments may be optionally provided:

• "single" enforces single quotes.
• "double" enforces double quotes.
• "backtick" enforces backticks.
• "jsx-single" enforces single quotes for JSX attributes.
• "jsx-double" enforces double quotes for JSX attributes.
• "avoid-template" forbids single-line untagged template strings that do not contain string interpolations. Note that backticks may still be used if "avoid-escape" is enabled and both single and double quotes are present in the string (the latter option takes precedence).
• "avoid-escape" allows you to use the "other" quotemark in cases where escaping would normally be required. For example, [true, "double", "avoid-escape"] would not report a failure on the string literal 'Hello "World"'.

Examples

"quotemark": true,single,avoid-escape,avoid-template

"quotemark": true,single,jsx-double

Schema

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "single",
      "double",
      "backtick",
      "jsx-single",
      "jsx-double",
      "avoid-escape",
      "avoid-template"
    ]
  },
  "minLength": 0,
  "maxLength": 5
}

For more information see this page.

Rule: no-unused-expression

Disallows unused expression statements.

Unused expressions are expression statements which are not assignments or function calls (and thus usually no-ops).

Rationale

Detects potential errors where an assignment or function call was intended.

Config

Three arguments may be optionally provided:

• allow-fast-null-checks allows to use logical operators to perform fast null checks and perform method or function calls for side effects (e.g. e && e.preventDefault()).
• allow-new allows 'new' expressions for side effects (e.g. new ModifyGlobalState();.
• allow-tagged-template allows tagged templates for side effects (e.g. this.add\foo`;`.

Examples

"no-unused-expression": true

"no-unused-expression": true,allow-fast-null-checks

Schema

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "allow-fast-null-checks",
      "allow-new",
      "allow-tagged-template"
    ]
  },
  "minLength": 0,
  "maxLength": 3
}

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: semicolon

Enforces consistent semicolon usage at the end of every statement.

Notes

• Has Fix

Config

One of the following arguments must be provided:

• "always" enforces semicolons at the end of every statement.
• "never" disallows semicolons at the end of every statement except for when they are necessary.

The following arguments may be optionally provided:

• "ignore-interfaces" skips checking semicolons at the end of interface members.
• "ignore-bound-class-methods" skips checking semicolons at the end of bound class methods.
• "strict-bound-class-methods" disables any special handling of bound class methods and treats them as any other assignment. This option overrides "ignore-bound-class-methods".

Examples

"semicolon": true,always

"semicolon": true,never

"semicolon": true,always,ignore-interfaces

"semicolon": true,always,ignore-bound-class-methods

Schema

{
  "type": "array",
  "items": [
    {
      "type": "string",
      "enum": [
        "always",
        "never"
      ]
    },
    {
      "type": "string",
      "enum": [
        "ignore-interfaces"
      ]
    }
  ],
  "additionalItems": false
}

For more information see this page.

Rule: quotemark

Enforces quote character for string literals.

Notes

• Has Fix

Config

Five arguments may be optionally provided:

• "single" enforces single quotes.
• "double" enforces double quotes.
• "backtick" enforces backticks.
• "jsx-single" enforces single quotes for JSX attributes.
• "jsx-double" enforces double quotes for JSX attributes.
• "avoid-template" forbids single-line untagged template strings that do not contain string interpolations. Note that backticks may still be used if "avoid-escape" is enabled and both single and double quotes are present in the string (the latter option takes precedence).
• "avoid-escape" allows you to use the "other" quotemark in cases where escaping would normally be required. For example, [true, "double", "avoid-escape"] would not report a failure on the string literal 'Hello "World"'.

Examples

"quotemark": true,single,avoid-escape,avoid-template

"quotemark": true,single,jsx-double

Schema

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "single",
      "double",
      "backtick",
      "jsx-single",
      "jsx-double",
      "avoid-escape",
      "avoid-template"
    ]
  },
  "minLength": 0,
  "maxLength": 5
}

For more information see this page.

Rule: quotemark

Enforces quote character for string literals.

Notes

• Has Fix

Config

Five arguments may be optionally provided:

• "single" enforces single quotes.
• "double" enforces double quotes.
• "backtick" enforces backticks.
• "jsx-single" enforces single quotes for JSX attributes.
• "jsx-double" enforces double quotes for JSX attributes.
• "avoid-template" forbids single-line untagged template strings that do not contain string interpolations. Note that backticks may still be used if "avoid-escape" is enabled and both single and double quotes are present in the string (the latter option takes precedence).
• "avoid-escape" allows you to use the "other" quotemark in cases where escaping would normally be required. For example, [true, "double", "avoid-escape"] would not report a failure on the string literal 'Hello "World"'.

Examples

"quotemark": true,single,avoid-escape,avoid-template

"quotemark": true,single,jsx-double

Schema

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "single",
      "double",
      "backtick",
      "jsx-single",
      "jsx-double",
      "avoid-escape",
      "avoid-template"
    ]
  },
  "minLength": 0,
  "maxLength": 5
}

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: ban-comma-operator

Disallows the comma operator to be used.

Read more about the comma operator here.

Rationale

Using the comma operator can create a potential for many non-obvious bugs or lead to misunderstanding of code.

Examples

foo((bar, baz)); // evaluates to 'foo(baz)' because of the extra parens - confusing and not obvious

switch (foo) {
    case 1, 2: // equals 'case 2' - probably intended 'case 1: case2:'
        return true;
    case 3:
        return false;
}

let x = (y = 1, z = 2); // x is equal to 2 - this may not be immediately obvious.

Examples

"ban-comma-operator": true

For more information see this page.

Rule: quotemark

Enforces quote character for string literals.

Notes

• Has Fix

Config

Five arguments may be optionally provided:

• "single" enforces single quotes.
• "double" enforces double quotes.
• "backtick" enforces backticks.
• "jsx-single" enforces single quotes for JSX attributes.
• "jsx-double" enforces double quotes for JSX attributes.
• "avoid-template" forbids single-line untagged template strings that do not contain string interpolations. Note that backticks may still be used if "avoid-escape" is enabled and both single and double quotes are present in the string (the latter option takes precedence).
• "avoid-escape" allows you to use the "other" quotemark in cases where escaping would normally be required. For example, [true, "double", "avoid-escape"] would not report a failure on the string literal 'Hello "World"'.

Examples

"quotemark": true,single,avoid-escape,avoid-template

"quotemark": true,single,jsx-double

Schema

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "single",
      "double",
      "backtick",
      "jsx-single",
      "jsx-double",
      "avoid-escape",
      "avoid-template"
    ]
  },
  "minLength": 0,
  "maxLength": 5
}

For more information see this page.

Rule: quotemark

Enforces quote character for string literals.

Notes

• Has Fix

Config

Five arguments may be optionally provided:

• "single" enforces single quotes.
• "double" enforces double quotes.
• "backtick" enforces backticks.
• "jsx-single" enforces single quotes for JSX attributes.
• "jsx-double" enforces double quotes for JSX attributes.
• "avoid-template" forbids single-line untagged template strings that do not contain string interpolations. Note that backticks may still be used if "avoid-escape" is enabled and both single and double quotes are present in the string (the latter option takes precedence).
• "avoid-escape" allows you to use the "other" quotemark in cases where escaping would normally be required. For example, [true, "double", "avoid-escape"] would not report a failure on the string literal 'Hello "World"'.

Examples

"quotemark": true,single,avoid-escape,avoid-template

"quotemark": true,single,jsx-double

Schema

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "single",
      "double",
      "backtick",
      "jsx-single",
      "jsx-double",
      "avoid-escape",
      "avoid-template"
    ]
  },
  "minLength": 0,
  "maxLength": 5
}

For more information see this page.

Rule: quotemark

Enforces quote character for string literals.

Notes

• Has Fix

Config

Five arguments may be optionally provided:

• "single" enforces single quotes.
• "double" enforces double quotes.
• "backtick" enforces backticks.
• "jsx-single" enforces single quotes for JSX attributes.
• "jsx-double" enforces double quotes for JSX attributes.
• "avoid-template" forbids single-line untagged template strings that do not contain string interpolations. Note that backticks may still be used if "avoid-escape" is enabled and both single and double quotes are present in the string (the latter option takes precedence).
• "avoid-escape" allows you to use the "other" quotemark in cases where escaping would normally be required. For example, [true, "double", "avoid-escape"] would not report a failure on the string literal 'Hello "World"'.

Examples

"quotemark": true,single,avoid-escape,avoid-template

"quotemark": true,single,jsx-double

Schema

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "single",
      "double",
      "backtick",
      "jsx-single",
      "jsx-double",
      "avoid-escape",
      "avoid-template"
    ]
  },
  "minLength": 0,
  "maxLength": 5
}

For more information see this page.

Rule: quotemark

Enforces quote character for string literals.

Notes

• Has Fix

Config

Five arguments may be optionally provided:

• "single" enforces single quotes.
• "double" enforces double quotes.
• "backtick" enforces backticks.
• "jsx-single" enforces single quotes for JSX attributes.
• "jsx-double" enforces double quotes for JSX attributes.
• "avoid-template" forbids single-line untagged template strings that do not contain string interpolations. Note that backticks may still be used if "avoid-escape" is enabled and both single and double quotes are present in the string (the latter option takes precedence).
• "avoid-escape" allows you to use the "other" quotemark in cases where escaping would normally be required. For example, [true, "double", "avoid-escape"] would not report a failure on the string literal 'Hello "World"'.

Examples

"quotemark": true,single,avoid-escape,avoid-template

"quotemark": true,single,jsx-double

Schema

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "single",
      "double",
      "backtick",
      "jsx-single",
      "jsx-double",
      "avoid-escape",
      "avoid-template"
    ]
  },
  "minLength": 0,
  "maxLength": 5
}

For more information see this page.

Rule: quotemark

Enforces quote character for string literals.

Notes

• Has Fix

Config

Five arguments may be optionally provided:

• "single" enforces single quotes.
• "double" enforces double quotes.
• "backtick" enforces backticks.
• "jsx-single" enforces single quotes for JSX attributes.
• "jsx-double" enforces double quotes for JSX attributes.
• "avoid-template" forbids single-line untagged template strings that do not contain string interpolations. Note that backticks may still be used if "avoid-escape" is enabled and both single and double quotes are present in the string (the latter option takes precedence).
• "avoid-escape" allows you to use the "other" quotemark in cases where escaping would normally be required. For example, [true, "double", "avoid-escape"] would not report a failure on the string literal 'Hello "World"'.

Examples

"quotemark": true,single,avoid-escape,avoid-template

"quotemark": true,single,jsx-double

Schema

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "single",
      "double",
      "backtick",
      "jsx-single",
      "jsx-double",
      "avoid-escape",
      "avoid-template"
    ]
  },
  "minLength": 0,
  "maxLength": 5
}

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-unused-expression

Disallows unused expression statements.

Unused expressions are expression statements which are not assignments or function calls (and thus usually no-ops).

Rationale

Detects potential errors where an assignment or function call was intended.

Config

Three arguments may be optionally provided:

• allow-fast-null-checks allows to use logical operators to perform fast null checks and perform method or function calls for side effects (e.g. e && e.preventDefault()).
• allow-new allows 'new' expressions for side effects (e.g. new ModifyGlobalState();.
• allow-tagged-template allows tagged templates for side effects (e.g. this.add\foo`;`.

Examples

"no-unused-expression": true

"no-unused-expression": true,allow-fast-null-checks

Schema

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "allow-fast-null-checks",
      "allow-new",
      "allow-tagged-template"
    ]
  },
  "minLength": 0,
  "maxLength": 3
}

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: ban-comma-operator

Disallows the comma operator to be used.

Read more about the comma operator here.

Rationale

Using the comma operator can create a potential for many non-obvious bugs or lead to misunderstanding of code.

Examples

foo((bar, baz)); // evaluates to 'foo(baz)' because of the extra parens - confusing and not obvious

switch (foo) {
    case 1, 2: // equals 'case 2' - probably intended 'case 1: case2:'
        return true;
    case 3:
        return false;
}

let x = (y = 1, z = 2); // x is equal to 2 - this may not be immediately obvious.

Examples

"ban-comma-operator": true

For more information see this page.

Rule: quotemark

Enforces quote character for string literals.

Notes

• Has Fix

Config

Five arguments may be optionally provided:

• "single" enforces single quotes.
• "double" enforces double quotes.
• "backtick" enforces backticks.
• "jsx-single" enforces single quotes for JSX attributes.
• "jsx-double" enforces double quotes for JSX attributes.
• "avoid-template" forbids single-line untagged template strings that do not contain string interpolations. Note that backticks may still be used if "avoid-escape" is enabled and both single and double quotes are present in the string (the latter option takes precedence).
• "avoid-escape" allows you to use the "other" quotemark in cases where escaping would normally be required. For example, [true, "double", "avoid-escape"] would not report a failure on the string literal 'Hello "World"'.

Examples

"quotemark": true,single,avoid-escape,avoid-template

"quotemark": true,single,jsx-double

Schema

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "single",
      "double",
      "backtick",
      "jsx-single",
      "jsx-double",
      "avoid-escape",
      "avoid-template"
    ]
  },
  "minLength": 0,
  "maxLength": 5
}

For more information see this page.