Use nullish coalescing assignment in shell.js. NFC (#22660)

Author: sbc100

src/shell.js

@@ -317,20 +317,13 @@ if (ENVIRONMENT_IS_SHELL) {
     });
   };
 
-  if (typeof clearTimeout == 'undefined') {
-    globalThis.clearTimeout = (id) => {};
-  }
+  globalThis.clearTimeout ??= (id) => {};
 
-  if (typeof setTimeout == 'undefined') {
-    // spidermonkey lacks setTimeout but we use it above in readAsync.
-    globalThis.setTimeout = (f) => (typeof f == 'function') ? f() : abort();
-  }
+  // spidermonkey lacks setTimeout but we use it above in readAsync.
+  globalThis.setTimeout ??= (f) => (typeof f == 'function') ? f() : abort();
 
-  if (typeof scriptArgs != 'undefined') {
-    arguments_ = scriptArgs;
-  } else if (typeof arguments != 'undefined') {
-    arguments_ = arguments;
-  }
+  // v8 uses `arguments_` whereas spidermonkey uses `scriptArgs`
+  arguments_ = globalThis.arguments || globalThis.scriptArgs;
 
   if (typeof quit == 'function') {
     quit_ = (status, toThrow) => {
@@ -359,9 +352,9 @@ if (ENVIRONMENT_IS_SHELL) {
 
   if (typeof print != 'undefined') {
     // Prefer to use print/printErr where they exist, as they usually work better.
-    if (typeof console == 'undefined') console = /** @type{!Console} */({});
+    globalThis.console ??= /** @type{!Console} */({});
     console.log = /** @type{!function(this:Console, ...*): undefined} */ (print);
-    console.warn = console.error = /** @type{!function(this:Console, ...*): undefined} */ (typeof printErr != 'undefined' ? printErr : print);
+    console.warn = console.error = /** @type{!function(this:Console, ...*): undefined} */ (globalThis.printErr ?? print);
   }
 
 #if WASM == 2