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-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: no-shadowed-variable

Disallows shadowing variable declarations.

Rationale

When a variable in a local scope and a variable in the containing scope have the same name, shadowing occurs. Shadowing makes it impossible to access the variable in the containing scope and obscures to what value an identifier actually refers. Compare the following snippets:

const a = 'no shadow';
function print() {
    console.log(a);
}
print(); // logs 'no shadow'.

const a = 'no shadow';
function print() {
    const a = 'shadow'; // TSLint will complain here.
    console.log(a);
}
print(); // logs 'shadow'.

ESLint has an equivalent rule. For more background information, refer to this MDN closure doc.

Config

You can optionally pass an object to disable checking for certain kinds of declarations. Possible keys are "class", "enum", "function", "import", "interface", "namespace", "typeAlias" and "typeParameter". You can also pass "underscore" to ignore variable names that begin with _. Just set the value to false for the check you want to disable. All checks default to true, i.e. are enabled by default. Note that you cannot disable variables and parameters.

The option "temporalDeadZone" defaults to true which shows errors when shadowing block scoped declarations in their temporal dead zone. When set to false parameters, classes, enums and variables declared with let or const are not considered shadowed if the shadowing occurs within their temporal dead zone.

The following example shows how the "temporalDeadZone" option changes the linting result:

function fn(value) {
    if (value) {
        const tmp = value; // no error on this line if "temporalDeadZone" is false
        return tmp;
    }
    let tmp = undefined;
    if (!value) {
        const tmp = value; // this line always contains an error
        return tmp;
    }
}

Examples

"no-shadowed-variable": true

"no-shadowed-variable": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "class": {
      "type": "boolean"
    },
    "enum": {
      "type": "boolean"
    },
    "function": {
      "type": "boolean"
    },
    "import": {
      "type": "boolean"
    },
    "interface": {
      "type": "boolean"
    },
    "namespace": {
      "type": "boolean"
    },
    "typeAlias": {
      "type": "boolean"
    },
    "typeParameter": {
      "type": "boolean"
    },
    "temporalDeadZone": {
      "type": "boolean"
    },
    "underscore": {
      "type": "boolean"
    }
  }
}

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: 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: 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: 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: 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: 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: 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: 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-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-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-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-shadowed-variable

Disallows shadowing variable declarations.

Rationale

When a variable in a local scope and a variable in the containing scope have the same name, shadowing occurs. Shadowing makes it impossible to access the variable in the containing scope and obscures to what value an identifier actually refers. Compare the following snippets:

const a = 'no shadow';
function print() {
    console.log(a);
}
print(); // logs 'no shadow'.

const a = 'no shadow';
function print() {
    const a = 'shadow'; // TSLint will complain here.
    console.log(a);
}
print(); // logs 'shadow'.

ESLint has an equivalent rule. For more background information, refer to this MDN closure doc.

Config

You can optionally pass an object to disable checking for certain kinds of declarations. Possible keys are "class", "enum", "function", "import", "interface", "namespace", "typeAlias" and "typeParameter". You can also pass "underscore" to ignore variable names that begin with _. Just set the value to false for the check you want to disable. All checks default to true, i.e. are enabled by default. Note that you cannot disable variables and parameters.

The option "temporalDeadZone" defaults to true which shows errors when shadowing block scoped declarations in their temporal dead zone. When set to false parameters, classes, enums and variables declared with let or const are not considered shadowed if the shadowing occurs within their temporal dead zone.

The following example shows how the "temporalDeadZone" option changes the linting result:

function fn(value) {
    if (value) {
        const tmp = value; // no error on this line if "temporalDeadZone" is false
        return tmp;
    }
    let tmp = undefined;
    if (!value) {
        const tmp = value; // this line always contains an error
        return tmp;
    }
}

Examples

"no-shadowed-variable": true

"no-shadowed-variable": true,[object Object]

Schema

{
  "type": "object",
  "properties": {
    "class": {
      "type": "boolean"
    },
    "enum": {
      "type": "boolean"
    },
    "function": {
      "type": "boolean"
    },
    "import": {
      "type": "boolean"
    },
    "interface": {
      "type": "boolean"
    },
    "namespace": {
      "type": "boolean"
    },
    "typeAlias": {
      "type": "boolean"
    },
    "typeParameter": {
      "type": "boolean"
    },
    "temporalDeadZone": {
      "type": "boolean"
    },
    "underscore": {
      "type": "boolean"
    }
  }
}

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: 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: 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: 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-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: 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-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: 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: 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.