Require that interface names should or should not prefixed with I (interface-name-prefix)

Interfaces often represent important software contracts, so it can be helpful to prefix their names with I. The unprefixed name is then available for a class that provides a standard implementation of the interface. Alternatively, the contributor guidelines for the TypeScript repo suggest never prefixing interfaces with I.

Rule Details

This rule enforces whether or not the I prefix is required for interface names. The _ prefix is sometimes used to designate a private declaration, so the rule also supports a private interface that might be named _IAnimal instead of IAnimal.

Options

This rule has an object option:

• { "prefixWithI": "never" }: (default) disallows all interfaces being prefixed with "I" or "_I"
• { "prefixWithI": "always" }: requires all interfaces be prefixed with "I" (but does not allow "_I")
• { "prefixWithI": "always", "allowUnderscorePrefix": true }: requires all interfaces be prefixed with either "I" or "_I"

For backwards compatibility, this rule supports a string option instead:

• "never": Equivalent to { "prefixWithI": "never" }
• "always": Equivalent to { "prefixWithI": "always" }

Examples

never

Configuration: { "prefixWithI": "never" }

The following patterns are considered warnings:

interface IAnimal {
  name: string;
}

interface IIguana {
  name: string;
}

interface _IAnimal {
  name: string;
}

The following patterns are not warnings:

interface Animal {
  name: string;
}

interface Iguana {
  name: string;
}

always

Configuration: { "prefixWithI": "always" }

The following patterns are considered warnings:

interface Animal {
  name: string;
}

interface Iguana {
  name: string;
}

interface _IAnimal {
  name: string;
}

The following patterns are not warnings:

interface IAnimal {
  name: string;
}

interface IIguana {
  name: string;
}

always and allowing underscores

Configuration: { "prefixWithI": "always", "allowUnderscorePrefix": true }

The following patterns are considered warnings:

interface Animal {
  name: string;
}

interface Iguana {
  name: string;
}

The following patterns are not warnings:

interface IAnimal {
  name: string;
}

interface IIguana {
  name: string;
}

interface _IAnimal {
  name: string;
}

When Not To Use It

If you do not want to enforce interface name prefixing.

Further Reading

TypeScript Interfaces

Compatibility

TSLint: interface-name