Prefer instanceof Cmd rather than calling isCmd() with *one* argument

Unless you actually need to check that something is both a `Cmd` and also of the *correct* type, using `instanceof Cmd` directly should be a tiny bit more efficient since it avoids one function call and an unnecessary `undefined` check.

This patch uses ESLint to enforce this, since we obviously still want to keep the `isCmd` helper function for where it makes sense.

.eslintrc

@@ -179,6 +179,10 @@
         "selector": "CallExpression[callee.name='assert'][arguments.length!=2]",
         "message": "`assert()` must always be invoked with two arguments.",
       },
+      {
+        "selector": "CallExpression[callee.name='isCmd'][arguments.length<2]",
+        "message": "Use `instanceof Cmd` rather than `isCmd()` with one argument.",
+      },
       {
         "selector": "NewExpression[callee.name='Cmd']",
         "message": "Use `Cmd.get()` rather than `new Cmd()`.",

src/core/cmap.js

@@ -20,7 +20,7 @@ import {
   unreachable,
   warn,
 } from "../shared/util.js";
-import { EOF, isCmd, isName } from "./primitives.js";
+import { Cmd, EOF, isCmd, isName } from "./primitives.js";
 import { BaseStream } from "./base_stream.js";
 import { Lexer } from "./parser.js";
 import { MissingDataException } from "./core_utils.js";
@@ -920,7 +920,7 @@ const CMapFactory = (function CMapFactoryClosure() {
             parseCMapName(cMap, lexer);
           }
           previous = obj;
-        } else if (isCmd(obj)) {
+        } else if (obj instanceof Cmd) {
           switch (obj.cmd) {
             case "endcmap":
               break objLoop;

test/unit/primitives_spec.js

@@ -496,6 +496,8 @@ describe("primitives", function () {
   });
 
   describe("isCmd", function () {
+    /* eslint-disable no-restricted-syntax */
+
     it("handles non-commands", function () {
       const nonCmd = {};
       expect(isCmd(nonCmd)).toEqual(false);
@@ -511,6 +513,8 @@ describe("primitives", function () {
       expect(isCmd(cmd, "BT")).toEqual(true);
       expect(isCmd(cmd, "ET")).toEqual(false);
     });
+
+    /* eslint-enable no-restricted-syntax */
   });
 
   describe("isDict", function () {