parcel-bundler/parcel

Module scripts loaded with low priority when compiled

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#6,981 opened on Sep 23, 2021

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:bug: BugGood First Issue✨ Parcel 2

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Description

Native module scripts (e.g. <script type="module">) are typically loaded by browsers with high priority. However, when targeting browsers without native support, Parcel inserts <script defer nomodule> tags, which are loaded with low priority. See https://addyosmani.com/blog/script-priorities/.

With that said, these scripts still need the defer attribute to emulate the execution timing of native modules. One workaround to boost priority of deferred scripts is to also insert a <link rel="preload" as="script"> into the document head with an href with the same value as the script's src.

We should consider adding these link tags as well when including js targeted at these environments.

Related: #6980

Contributor guide

Research direction
Investigate Parcel's HTML bundler to find where script tags are generated and add logic to insert preload links for deferred module scripts.
Tech stack
javascripthtmlnodejs
Domain
frontendbuild system
Issue type
Feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Half day
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
JavaScriptHTMLParcel
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40