add ObjectArgument type

Author: bouzuya

src/rules/converters/space-before-function-paren.ts

@@ -2,19 +2,24 @@ import { RuleConverter } from "../converter";
 
 const SUPPORTED_OPTIONS: string[] = ["anonymous", "asyncArrow", "named"];
 
+type ObjectArgument = Record<string, "always" | "never">;
+
+const isObjectArgument = (argument: unknown): argument is ObjectArgument =>
+    typeof argument === "object" && argument !== null;
+
 export const convertSpaceBeforeFunctionParen: RuleConverter = tslintRule => {
-    const argument: "always" | "never" | { [key: string]: "always" | "never" } | undefined =
-        tslintRule.ruleArguments[0];
+    const argument: unknown = tslintRule.ruleArguments[0];
     const ruleName = "space-before-function-paren";
     if (argument === "always" || argument === "never") {
         return { rules: [{ ruleArguments: [argument], ruleName }] };
-    } else if (typeof argument === "object" && argument !== null) {
+    }
+    if (isObjectArgument(argument)) {
         const notices = Object.keys(argument)
             .filter(option => !SUPPORTED_OPTIONS.includes(option))
             .map(option => `Option "${option}" is not supported by ESLint.`);
         const filtered = Object.keys(argument)
             .filter(option => SUPPORTED_OPTIONS.includes(option))
-            .reduce<Record<string, "always" | "never">>((obj, option) => {
+            .reduce<ObjectArgument>((obj, option) => {
                 return { ...obj, [option]: argument[option] };
             }, {});
         return {