fix(eslint-plugin): [no-base-to-string] support boolean in unions (typescript-eslint#1979)

auhsoja · web-flow · commit 6987ecc1dacf · 2020-05-10T18:49:41.000-07:00
diff --git a/packages/eslint-plugin/src/rules/no-base-to-string.ts b/packages/eslint-plugin/src/rules/no-base-to-string.ts
@@ -123,10 +123,27 @@ export default util.createRule<Options, MessageIds>({
         return Usefulness.Never;
       }
 
+      let allSubtypesUseful = true;
+      let someSubtypeUseful = false;
+
       for (const subType of type.types) {
-        if (collectToStringCertainty(subType) !== Usefulness.Never) {
-          return Usefulness.Sometimes;
+        const subtypeUsefulness = collectToStringCertainty(subType);
+
+        if (subtypeUsefulness !== Usefulness.Always && allSubtypesUseful) {
+          allSubtypesUseful = false;
         }
+
+        if (subtypeUsefulness !== Usefulness.Never && !someSubtypeUseful) {
+          someSubtypeUseful = true;
+        }
+      }
+
+      if (allSubtypesUseful && someSubtypeUseful) {
+        return Usefulness.Always;
+      }
+
+      if (someSubtypeUseful) {
+        return Usefulness.Sometimes;
       }
 
       return Usefulness.Never;
diff --git a/packages/eslint-plugin/tests/rules/no-base-to-string.test.ts b/packages/eslint-plugin/tests/rules/no-base-to-string.test.ts
@@ -78,6 +78,14 @@ const literalWithToString = {
   toString: () => 'Hello, world!',
 };
 '' + literalToString;
+    `,
+    `
+const printer = (inVar: string | number | boolean) => {
+  inVar.toString();
+};
+printer('');
+printer(1);
+printer(true);
     `,
     'let _ = {} * {};',
     'let _ = {} / {};',
