I posted this on #clojurescript but didn’t get much input and it’s possible it could be relevant here instead, given the compiler-side sort of expertise here: When upgrading from clojurescript v 1.10.764 to 1.10.866 , which notably had 2 goog closure lib upgrades I note with a dev figwheel style build (latest lein-figwheel + cljsbuild) that I suddenly get

Uncaught SyntaxError: Unexpected token '!'

/*TRANSPILED*//*

 Copyright The Closure Library Authors.
 SPDX-License-Identifier: Apache-2.0
*/

/*TRANSPILED*//*

 Copyright The Closure Library Authors.
 SPDX-License-Identifier: Apache-2.0
*/
goog.provide("goog.log");
goog.provide("goog.log.Level");
goog.provide("goog.log.LogBuffer");
goog.provide("goog.log.LogRecord");
goog.provide("goog.log.Logger");
goog.require("goog.asserts");
goog.require("goog.debug");
goog.log.Loggable;
goog.log.ENABLED = goog.define("goog.log.ENABLED", goog.debug.LOGGING_ENABLED);
goog.log.ROOT_LOGGER_NAME = "";
var file_$Users$myuser$_m2$repository$org$clojure$google_closure_library$0_0_20201211_3e6c510d$google_closure_library_0_0_20201211_3e6c510d_jar!$goog$log$log$classdecl$var0 = function(name, value) {
  this.name = name;
  this.value = value;
};