3 files changed, 67 insertions, 0 deletions
diff --git a/packages/tslint-config/CHANGELOG.md b/packages/tslint-config/CHANGELOG.md
index 45c9eaad8..31f49eef6 100644
--- a/packages/tslint-config/CHANGELOG.md
+++ b/packages/tslint-config/CHANGELOG.md
@@ -1,5 +1,10 @@
 # CHANGELOG
 
+v0.x.x - TBD
+------------------------
+    * Added custom 'underscore-privates' rule, requiring underscores to be prepended to private variable names
+    * Because our tools can be used in both a TS and JS environment, we want to make the private methods of any public facing interface show up at the bottom of auto-complete lists. Additionally, we wanted to remain consistent with respect to our usage of underscores in order to enforce this rule with a linter rule, rather then manual code reviews.
+
 v0.3.0 - _December 20, 2017_
 ------------------------
     * Added rules for unused imports, variables and Async suffixes (#265)
diff --git a/packages/tslint-config/rules/underscorePrivatesRule.ts b/packages/tslint-config/rules/underscorePrivatesRule.ts
new file mode 100644
index 000000000..472ea09ff
--- /dev/null
+++ b/packages/tslint-config/rules/underscorePrivatesRule.ts
@@ -0,0 +1,61 @@
+import * as Lint from 'tslint';
+import * as ts from 'typescript';
+
+const UNDERSCORE = '_';
+
+type RelevantClassMember =
+    | ts.MethodDeclaration
+    | ts.PropertyDeclaration
+    | ts.GetAccessorDeclaration
+    | ts.SetAccessorDeclaration;
+
+// Copied from: https://github.com/DanielRosenwasser/underscore-privates-tslint-rule
+// The version on github is not published on npm
+export class Rule extends Lint.Rules.AbstractRule {
+    public static FAILURE_STRING = 'private members must be prefixed with an underscore';
+
+    public apply(sourceFile: ts.SourceFile): Lint.RuleFailure[] {
+        return this.applyWithFunction(sourceFile, walk);
+    }
+}
+function walk(ctx: Lint.WalkContext<void>): void {
+    traverse(ctx.sourceFile);
+
+    function traverse(node: ts.Node): void {
+        checkNodeForViolations(ctx, node);
+        return ts.forEachChild(node, traverse);
+    }
+}
+function checkNodeForViolations(ctx: Lint.WalkContext<void>, node: ts.Node): void {
+    if (!isRelevantClassMember(node)) {
+        return;
+    }
+    // The declaration might have a computed property name or a numeric name.
+    const name = node.name;
+    if (!nameIsIdentifier(name)) {
+        return;
+    }
+    if (!nameStartsWithUnderscore(name.text) && memberIsPrivate(node)) {
+        ctx.addFailureAtNode(name, Rule.FAILURE_STRING);
+    }
+}
+function isRelevantClassMember(node: ts.Node): node is RelevantClassMember {
+    switch (node.kind) {
+        case ts.SyntaxKind.MethodDeclaration:
+        case ts.SyntaxKind.PropertyDeclaration:
+        case ts.SyntaxKind.GetAccessor:
+        case ts.SyntaxKind.SetAccessor:
+            return true;
+        default:
+            return false;
+    }
+}
+function nameStartsWithUnderscore(text: string) {
+    return text.charCodeAt(0) === UNDERSCORE.charCodeAt(0);
+}
+function memberIsPrivate(node: ts.Declaration) {
+    return Lint.hasModifier(node.modifiers, ts.SyntaxKind.PrivateKeyword);
+}
+function nameIsIdentifier(node: ts.Node): node is ts.Identifier {
+    return node.kind === ts.SyntaxKind.Identifier;
+}
diff --git a/packages/tslint-config/tslint.json b/packages/tslint-config/tslint.json
index acb6876c3..49e31b13d 100644
--- a/packages/tslint-config/tslint.json
+++ b/packages/tslint-config/tslint.json
@@ -81,6 +81,7 @@
     ],
     "space-within-parens": false,
     "type-literal-delimiter": true,
+    "underscore-privates": true,
     "variable-name": [true,
       "ban-keywords",
       "allow-pascal-case"