fix #149; avoid ES2020 syntax; enforce ES2019

Author: mbostock

eslint.config.js

@@ -4,5 +4,6 @@ import {defineConfig} from "eslint/config";
 
 export default defineConfig([
   {files: ["**/*.{js,ts,jsx,tsx}"], plugins: {js}, extends: ["js/recommended"]},
-  {files: ["**/*.{js,ts,jsx,tsx}"], languageOptions: {globals: globals.browser}}
+  {files: ["**/*.{js,ts,jsx,tsx}"], languageOptions: {globals: globals.browser}},
+  {languageOptions: {ecmaVersion: 2019, sourceType: "module"}}
 ]);

src/locale.js

@@ -45,8 +45,8 @@ export default function(locale) {
 
     // Compute the prefix and suffix.
     // For SI-prefix, the suffix is lazily computed.
-    var prefix = (options?.prefix !== undefined ? options.prefix : "") + (symbol === "$" ? currencyPrefix : symbol === "#" && /[boxX]/.test(type) ? "0" + type.toLowerCase() : ""),
-        suffix = (symbol === "$" ? currencySuffix : /[%p]/.test(type) ? percent : "") + (options?.suffix !== undefined ? options.suffix : "");
+    var prefix = (options && options.prefix !== undefined ? options.prefix : "") + (symbol === "$" ? currencyPrefix : symbol === "#" && /[boxX]/.test(type) ? "0" + type.toLowerCase() : ""),
+        suffix = (symbol === "$" ? currencySuffix : /[%p]/.test(type) ? percent : "") + (options && options.suffix !== undefined ? options.suffix : "");
 
     // What format function should we use?
     // Is this an integer type?