Change isNodeJS from a function to a constant

Given that this shouldn't change after the `pdf.js`/`pdf.worker.js` files have been loaded, it doesn't seems necessary to keep this as a function.

src/core/worker.js

@@ -581,7 +581,7 @@ function isMessagePort(maybePort) {
 }
 
 // Worker thread (and not node.js)?
-if (typeof window === 'undefined' && !isNodeJS() &&
+if (typeof window === 'undefined' && !isNodeJS &&
     typeof self !== 'undefined' && isMessagePort(self)) {
   WorkerMessageHandler.initializeFromPort(self);
 }

src/display/api_compatibility.js

@@ -35,7 +35,7 @@ if (typeof PDFJSDev === 'undefined' || PDFJSDev.test('GENERIC')) {
   // Support: Node.js
   (function checkFontFaceAndImage() {
     // Node.js is missing native support for `@font-face` and `Image`.
-    if (isNodeJS()) {
+    if (isNodeJS) {
       compatibilityParams.disableFontFace = true;
       compatibilityParams.nativeImageDecoderSupport = 'none';
     }

src/display/svg.js

@@ -109,7 +109,7 @@ const convertImgDataToPng = (function() {
    *   http://www.libpng.org/pub/png/spec/1.2/PNG-Compression.html
    */
   function deflateSync(literals) {
-    if (!isNodeJS()) {
+    if (!isNodeJS) {
       // zlib is certainly not available outside of Node.js. We can either use
       // the pako library for client-side DEFLATE compression, or use the canvas
       // API of the browser to obtain a more optimal PNG file.

src/pdf.js

@@ -32,7 +32,7 @@ let pdfjsDisplayAPICompatibility = require('./display/api_compatibility.js');
 
 if (typeof PDFJSDev === 'undefined' || PDFJSDev.test('GENERIC')) {
   const { isNodeJS, } = require('./shared/is_node.js');
-  if (isNodeJS()) {
+  if (isNodeJS) {
     let PDFNodeStream = require('./display/node_stream.js').PDFNodeStream;
     pdfjsDisplayAPI.setPDFNetworkStreamFactory((params) => {
       return new PDFNodeStream(params);

src/shared/compatibility.js

@@ -28,7 +28,7 @@ const hasDOM = typeof window === 'object' && typeof document === 'object';
 
 // Support: Node.js
 (function checkNodeBtoa() {
-  if (globalScope.btoa || !isNodeJS()) {
+  if (globalScope.btoa || !isNodeJS) {
     return;
   }
   globalScope.btoa = function(chars) {
@@ -39,7 +39,7 @@ const hasDOM = typeof window === 'object' && typeof document === 'object';
 
 // Support: Node.js
 (function checkNodeAtob() {
-  if (globalScope.atob || !isNodeJS()) {
+  if (globalScope.atob || !isNodeJS) {
     return;
   }
   globalScope.atob = function(input) {
@@ -69,7 +69,7 @@ const hasDOM = typeof window === 'object' && typeof document === 'object';
 // one parameter, in legacy browsers.
 // Support: IE
 (function checkDOMTokenListAddRemove() {
-  if (!hasDOM || isNodeJS()) {
+  if (!hasDOM || isNodeJS) {
     return;
   }
   const div = document.createElement('div');
@@ -98,7 +98,7 @@ const hasDOM = typeof window === 'object' && typeof document === 'object';
 // "force" parameter, in legacy browsers.
 // Support: IE
 (function checkDOMTokenListToggle() {
-  if (!hasDOM || isNodeJS()) {
+  if (!hasDOM || isNodeJS) {
     return;
   }
   const div = document.createElement('div');

src/shared/is_node.js

@@ -14,14 +14,13 @@
  */
 /* globals process */
 
-function isNodeJS() {
-  // NW.js / Electron is a browser context, but copies some Node.js objects; see
-  // http://docs.nwjs.io/en/latest/For%20Users/Advanced/JavaScript%20Contexts%20in%20NW.js/#access-nodejs-and-nwjs-api-in-browser-context
-  // https://electronjs.org/docs/api/process#processversionselectron
-  return typeof process === 'object' &&
-         process + '' === '[object process]' &&
-         !process.versions['nw'] && !process.versions['electron'];
-}
+// NW.js / Electron is a browser context, but copies some Node.js objects; see
+// http://docs.nwjs.io/en/latest/For%20Users/Advanced/JavaScript%20Contexts%20in%20NW.js/#access-nodejs-and-nwjs-api-in-browser-context
+// https://electronjs.org/docs/api/process#processversionselectron
+const isNodeJS =
+  typeof process === 'object' &&
+  process + '' === '[object process]' &&
+  !process.versions['nw'] && !process.versions['electron'];
 
 export {
   isNodeJS,