fix #3453: use Symbol.for for missing symbols

CHANGELOG.md

@@ -60,6 +60,10 @@
     }
     ```
 
+* No longer throw an error when a `Symbol` is missing ([#3453](https://github.com/evanw/esbuild/issues/3453))
+
+    Certain JavaScript syntax features use special properties on the global `Symbol` object. For example, the asynchronous iteration syntax uses `Symbol.asyncIterator`. Previously esbuild's generated code for older browsers required this symbol to be polyfilled. However, starting with this release esbuild will use [`Symbol.for()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol/for) to construct these symbols if they are missing instead of throwing an error about a missing polyfill. This means your code no longer needs to include a polyfill for missing symbols as long as your code also uses `Symbol.for()` for missing symbols.
+
 * Parse upcoming changes to TypeScript syntax ([#3490](https://github.com/evanw/esbuild/issues/3490), [#3491](https://github.com/evanw/esbuild/pull/3491))
 
     With this release, you can now use `from` as the name of a default type-only import in TypeScript code, as well as `of` as the name of an `await using` loop iteration variable:

internal/runtime/runtime.go

@@ -76,8 +76,7 @@ func Source(unsupportedJSFeatures compat.JSFeature) logger.Source {
 		var __reflectSet = Reflect.set
 
 		var __knownSymbol = (name, symbol) => {
-			if (symbol = Symbol[name]) return symbol
-			throw Error('Symbol.' + name + ' is not defined')
+			return (symbol = Symbol[name]) ? symbol : Symbol.for('Symbol.' + name)
 		}
 
 		export var __pow = Math.pow
@@ -383,7 +382,7 @@ func Source(unsupportedJSFeatures compat.JSFeature) logger.Source {
 			var method = k => it[k] = x => new Promise((yes, no) => resume(k, x, yes, no))
 			var it = {}
 			return generator = generator.apply(__this, __arguments),
-				it[Symbol.asyncIterator] = () => it,
+				it[__knownSymbol('asyncIterator')] = () => it,
 				method('next'),
 				method('throw'),
 				method('return'),