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

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

Tech stack
javascripthtmlcss
Domain
frontendperformance
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.
mostly clear
Prerequisites
Basic understanding of Parcel bundlerKnowledge of script loading priorities
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
55
Research direction
Investigate how Parcel currently generates `<script defer nomodule>` tags for non module targets. Look at the bundler's HTML asset handling, likely in packages/core/integration tests or similar. Consider adding logic to insert `<link rel="preload" as="script">` for each deferred script. Related issue #6980 provides additional context.
Module scripts loaded with low priority when compiled · parcel-bundler/parcel#6981 | Good First Issue