url: refactor lib/internal/url.js

* set an identifier for the separator rather than using multiple
  instances of the same literal
* consistent arrow function body formatting

PR-URL: nodejs/node#10912
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>

lib/internal/url.js

@@ -862,6 +862,7 @@ URLSearchParams.prototype[Symbol.iterator] = URLSearchParams.prototype.entries;
 
 URLSearchParams.prototype[util.inspect.custom] =
   function inspect(recurseTimes, ctx) {
+    const separator = ', ';
     const innerOpts = Object.assign({}, ctx);
     if (recurseTimes !== null) {
       innerOpts.depth = recurseTimes - 1;
@@ -874,13 +875,14 @@ URLSearchParams.prototype[util.inspect.custom] =
       output.push(`${innerInspect(list[i])} => ${innerInspect(list[i + 1])}`);
 
     const colorRe = /\u001b\[\d\d?m/g;
-    const length = output.reduce((prev, cur) => {
-      return prev + cur.replace(colorRe, '').length + ', '.length;
-    }, -', '.length);
+    const length = output.reduce(
+      (prev, cur) => prev + cur.replace(colorRe, '').length + separator.length,
+      -separator.length
+    );
     if (length > ctx.breakLength) {
       return `${this.constructor.name} {\n  ${output.join(',\n  ')} }`;
     } else if (output.length) {
-      return `${this.constructor.name} { ${output.join(', ')} }`;
+      return `${this.constructor.name} { ${output.join(separator)} }`;
     } else {
       return `${this.constructor.name} {}`;
     }