typescript-eslint · JamesHenry · Sep 12, 2019 · Aug 2, 2019 · Aug 3, 2019 · Aug 13, 2019
diff --git a/packages/eslint-plugin/docs/rules/strict-boolean-expressions.md b/packages/eslint-plugin/docs/rules/strict-boolean-expressions.md
@@ -56,6 +56,7 @@ while (typeof str !== 'undefined') {
 
 Options may be provided as an object with:
 
+- `allowNullable` to allow `undefined` and `null` in addition to `boolean` as a type of all boolean expressions. (`false` by default).
 - `ignoreRhs` to skip the check on the right hand side of expressions like `a && b` or `a || b` - allows these operators to be used for their short-circuiting behavior. (`false` by default).
 
 ## Related To

diff --git a/packages/eslint-plugin/src/rules/strict-boolean-expressions.ts b/packages/eslint-plugin/src/rules/strict-boolean-expressions.ts
@@ -16,6 +16,7 @@ type ExpressionWithTest =
 type Options = [
   {
     ignoreRhs?: boolean;
+    allowNullable?: boolean;
   },
 ];
 
@@ -36,6 +37,9 @@ export default util.createRule<Options, 'strictBooleanExpression'>({
           ignoreRhs: {
             type: 'boolean',
           },
+          allowNullable: {
+            type: 'boolean',
+          },
         },
         additionalProperties: false,
       },
@@ -47,9 +51,10 @@ export default util.createRule<Options, 'strictBooleanExpression'>({
   defaultOptions: [
     {
       ignoreRhs: false,
+      allowNullable: false,
     },
   ],
-  create(context, [{ ignoreRhs }]) {
+  create(context, [options]) {
     const service = util.getParserServices(context);
     const checker = service.program.getTypeChecker();
 
@@ -61,7 +66,34 @@ export default util.createRule<Options, 'strictBooleanExpression'>({
         node,
       );
       const type = util.getConstrainedTypeAtLocation(checker, tsNode);
-      return tsutils.isTypeFlagSet(type, ts.TypeFlags.BooleanLike);
+
+      if (tsutils.isTypeFlagSet(type, ts.TypeFlags.BooleanLike)) {
+        return true;
+      }
+
+      // Check variants of union
+      if (tsutils.isTypeFlagSet(type, ts.TypeFlags.Union)) {
+        let hasBoolean = false;
+        for (const ty of (type as ts.UnionType).types) {
+          if (tsutils.isTypeFlagSet(ty, ts.TypeFlags.BooleanLike)) {
+            hasBoolean = true;
+            continue;
+          }
+          if (options.allowNullable) {
+            if (tsutils.isTypeFlagSet(ty, ts.TypeFlags.Null)) {
+              continue;
+            }
+            if (tsutils.isTypeFlagSet(ty, ts.TypeFlags.Undefined)) {
+              continue;
+            }
+          }
+          // Union variant is something else
+          return false;
+        }
+        return hasBoolean;
+      }
+
+      return false;
     }
 
     /**
@@ -88,7 +120,7 @@ export default util.createRule<Options, 'strictBooleanExpression'>({
     ): void {
       if (
         !isBooleanType(node.left) ||
-        (!ignoreRhs && !isBooleanType(node.right))
+        (!options.ignoreRhs && !isBooleanType(node.right))
       ) {
         reportNode(node);
       }

diff --git a/packages/eslint-plugin/tests/rules/strict-boolean-expressions.test.ts b/packages/eslint-plugin/tests/rules/strict-boolean-expressions.test.ts
@@ -165,6 +165,15 @@ const boolOrObj = bool || obj;
 const boolAndObj = bool && obj;
 `,
     },
+    {
+      options: [{ allowNullable: true }],
+      code: `
+        const f1 = (x?: boolean) => x ? 1 : 0;
+        const f2 = (x: boolean | null) => x ? 1 : 0;
+        const f3 = (x?: true | null) => x ? 1 : 0;
+        const f4 = (x?: false) => x ? 1 : 0;
+      `,
+    },
   ],
 
   invalid: [
@@ -933,5 +942,24 @@ const objOrBool = obj || bool;
 const objAndBool = obj && bool;
 `,
     },
+    {
+      options: [{ allowNullable: true }],
+      errors: [
+        {
+          messageId: 'strictBooleanExpression',
+          line: 2,
+          column: 44,
+        },
+        {
+          messageId: 'strictBooleanExpression',
+          line: 3,
+          column: 35,
+        },
+      ],
+      code: `
+        const f = (x: null | undefined) => x ? 1 : 0;
+        const f = (x?: number) => x ? 1 : 0;
+      `,
+    },
   ],
 });