Rule: triple-equals

Requires === and !== in place of == and !=.

Config

Two arguments may be optionally provided:

• "allow-null-check" allows == and != when comparing to null.
• "allow-undefined-check" allows == and != when comparing to undefined.

Examples

"triple-equals": true

"triple-equals": true,allow-null-check

"triple-equals": true,allow-undefined-check

Schema

{
  "type": "array",
  "items": {
    "type": "string",
    "enum": [
      "allow-null-check",
      "allow-undefined-check"
    ]
  },
  "minLength": 0,
  "maxLength": 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: 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: comment-format

Enforces formatting rules for single-line comments.

Rationale

Helps maintain a consistent, readable style in your codebase.

Notes

• Has Fix

Config

Four arguments may be optionally provided:

• "check-space" requires that all single-line comments must begin with a space, as in // comment
  • note that for comments starting with multiple slashes, e.g. ///, leading slashes are ignored
  • TypeScript reference comments are ignored completely
• "check-lowercase" requires that the first non-whitespace character of a comment must be lowercase, if applicable.
• "check-uppercase" requires that the first non-whitespace character of a comment must be uppercase, if applicable.
• "allow-trailing-lowercase" allows that only the first comment of a series of comments needs to be uppercase.
  • requires "check-uppercase"
  • comments must start at the same position

Exceptions to "check-lowercase" or "check-uppercase" can be managed with object that may be passed as last argument.

One of two options can be provided in this object:

• "ignore-words" - array of strings - words that will be ignored at the beginning of the comment.
• "ignore-pattern" - string - RegExp pattern that will be ignored at the beginning of the comment.

Examples

"comment-format": true,check-space,check-uppercase,allow-trailing-lowercase

"comment-format": true,check-lowercase,[object Object]

"comment-format": true,check-lowercase,[object Object]

Schema

{
  "type": "array",
  "items": {
    "anyOf": [
      {
        "type": "string",
        "enum": [
          "check-space",
          "check-lowercase",
          "check-uppercase",
          "allow-trailing-lowercase"
        ]
      },
      {
        "type": "object",
        "properties": {
          "ignore-words": {
            "type": "array",
            "items": {
              "type": "string"
            }
          },
          "ignore-pattern": {
            "type": "string"
          }
        },
        "minProperties": 1,
        "maxProperties": 1
      }
    ]
  },
  "minLength": 1,
  "maxLength": 5
}

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: 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-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: 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.