From 62b2b1f1f9208615b8c25c1555abd8f3f37040e0 Mon Sep 17 00:00:00 2001
From: Pete Gonzalez <4673363+octogonz@users.noreply.github.com>
Date: Wed, 31 Jul 2019 23:32:54 -0700
Subject: [PATCH] feat(eslint-plugin): Improve "interface-name-prefix" to
 accept a private name such as "_IAnimal"

---
 .../docs/rules/interface-name-prefix.md       | 37 ++++++++++++++++---
 .../src/rules/interface-name-prefix.ts        |  2 +-
 .../tests/rules/interface-name-prefix.test.ts |  8 ++++
 3 files changed, 41 insertions(+), 6 deletions(-)

diff --git a/packages/eslint-plugin/docs/rules/interface-name-prefix.md b/packages/eslint-plugin/docs/rules/interface-name-prefix.md
index c6d907568a1..1ef652dffd7 100644
--- a/packages/eslint-plugin/docs/rules/interface-name-prefix.md
+++ b/packages/eslint-plugin/docs/rules/interface-name-prefix.md
@@ -1,18 +1,21 @@
 # Require that interface names be prefixed with `I` (interface-name-prefix)
 
-It can be hard to differentiate between classes and interfaces.
-Prefixing interfaces with "I" can help telling them apart at a glance.
+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.
 
 ## Rule Details
 
-This rule enforces consistency of interface naming prefix conventions.
+This rule enforces whether or not the `I` prefix is required for interface names.
 
 ## Options
 
 This rule has a string option.
 
-- `"never"` (default) disallows all interfaces being prefixed with `"I"`
-- `"always"` requires all interfaces be prefixed with `"I"`
+- `"never"` (default) disallows all interfaces being prefixed with `"I"` (or `"_I"`)
+- `"always"` requires all interfaces be prefixed with `"I"` (or `"_I"`)
+
+The `_` prefix is sometimes used to designate a private declaration, in which case a private interface might be
+named `_IAnimal` instead of `IAnimal`.  The rule recognizes both forms.
 
 ### never
 
@@ -24,6 +27,14 @@ The following patterns are considered warnings:
 interface IAnimal {
   name: string;
 }
+
+interface _IAnimal {
+  name: string;
+}
+
+interface IIguana {
+  name: string;
+}
 ```
 
 The following patterns are not warnings:
@@ -32,6 +43,10 @@ The following patterns are not warnings:
 interface Animal {
   name: string;
 }
+
+interface Iguana {
+  name: string;
+}
 ```
 
 ### always
@@ -42,6 +57,10 @@ The following patterns are considered warnings:
 interface Animal {
   name: string;
 }
+
+interface Iguana {
+  name: string;
+}
 ```
 
 The following patterns are not warnings:
@@ -50,6 +69,14 @@ The following patterns are not warnings:
 interface IAnimal {
   name: string;
 }
+
+interface _IAnimal {
+  name: string;
+}
+
+interface IIguana {
+  name: string;
+}
 ```
 
 ## When Not To Use It
diff --git a/packages/eslint-plugin/src/rules/interface-name-prefix.ts b/packages/eslint-plugin/src/rules/interface-name-prefix.ts
index 1af78f79a53..f90e1557b6d 100644
--- a/packages/eslint-plugin/src/rules/interface-name-prefix.ts
+++ b/packages/eslint-plugin/src/rules/interface-name-prefix.ts
@@ -35,7 +35,7 @@ export default util.createRule<Options, MessageIds>({
         return false;
       }
 
-      return /^I[A-Z]/.test(name);
+      return /^_?I[A-Z]/.test(name);
     }
 
     return {
diff --git a/packages/eslint-plugin/tests/rules/interface-name-prefix.test.ts b/packages/eslint-plugin/tests/rules/interface-name-prefix.test.ts
index 09cf548c7ac..dd166e75833 100644
--- a/packages/eslint-plugin/tests/rules/interface-name-prefix.test.ts
+++ b/packages/eslint-plugin/tests/rules/interface-name-prefix.test.ts
@@ -22,6 +22,14 @@ interface IAnimal {
     },
     {
       code: `
+interface _IAnimal {
+    name: string;
+}
+            `,
+      options: ['always'],
+    },
+    {
+      code: `
 interface IIguana {
     name: string;
 }