Build system: adjust babel configuration to reduce bundle size

[dgirardi]

Type of change

• Build related changes

Description of change

This updates babel configuration to:

• not polyfill Object.assign (all browers we care about now support it)
• share other polyfills among source files, instead of redefining them each time they are needed.

The result is ~10KB reduction in prebid-core and ~150KB overall.

Before:

-rw-rw-r-- 1 demetrio demetrio  147195 Jul 22 15:29 build/dist/prebid-core.js
-rw-rw-r-- 1 demetrio demetrio 2455160 Jul 22 15:29 build/dist/prebid.js

After:

-rw-rw-r-- 1 demetrio demetrio  136981 Jul 22 15:27 build/dist/prebid-core.js
-rw-rw-r-- 1 demetrio demetrio 2306056 Jul 22 15:27 build/dist/prebid.js

[muuki88]

This seems to have broken our typescript build. During runtime we get the following error

Uncaught ReferenceError: exports is not defined

https://stackoverflow.com/questions/43042889/typescript-referenceerror-exports-is-not-defined

I'm trying to chase this down why it happens. Curious if others see this as well.

[muuki88]

I found: webpack/webpack#4039 (comment)

@babel/plugin-transform-runtime, and you are running Babel on CommonJS files (either your own, or in random node_modules, if you're trying to compile that). Babel assumes files are ES modules by default, meaning those transforms would insert import statements into your file, triggering case 1. above

I assume we have to detangle the babel configs in our build.

[muuki88]

Now I found another bug.

Prebid.js/src/secureCreatives.js

Lines 25 to 29 in a231b0d

	if (FEATURES.NATIVE) {
	Object.assign(HANDLER_MAP, {
	'Prebid Native': handleNativeRequest,
	})
	}

The FEATURES variable is not imported or required in anyway. Our webpack build with prebid as a npm dependency works, but during runtime we now get

secureCreatives.js:25 Uncaught ReferenceError: FEATURES is not defined
    at ./node_modules/prebid.js/src/secureCreatives.js (secureCreatives.js:25:1)
    at __webpack_require__ (bootstrap:19:1)
    at ./node_modules/prebid.js/src/prebid.js (polyfill.js:18:2)
    at __webpack_require__ (bootstrap:19:1)
    at make namespace object:7:1
    at moli.mjs:41251:3
    at moli.mjs:41253:12

This was introduced in #8219

--- Update

Maybe again related to our webpack build.