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Add examples of how module maps are keyed #2193

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Add examples of how module maps are keyed #2193

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Add examples of how module maps are keyed
domenic Dec 19, 2016
6cba2f9
Get rid of controversial example
domenic Dec 20, 2016
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Expand Up @@ -87754,7 +87754,38 @@ document.querySelector("button").addEventListener("click", bound);
URL">absolute URLs</span> to values that are either a <span>module script</span>, null (used to
represent failed fetches), or a placeholder value "<code data-x="">fetching</code>". <span
data-x="module map">Module maps</span> are used to ensure that imported JavaScript modules are
only fetched, parsed, and evaluated once per <code>Document</code> or <a href="#workers">worker</a>.</p>
only fetched, parsed, and evaluated once per <code>Document</code> or <a
href="#workers">worker</a>.</p>

<div class="example">
<p>Since <span data-x="module map">module maps</span> are keyed by URL, the following code will
create three separate entries in the <span>module map</span>, since it results in three different
URLs:</p>

<pre>import "https://example.com/module.js";
import "https://example.com/module.js#map-buster";
import "https://example.com/module.js?debug=true";</pre>
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I think currently in Chrome this will be normalized. I currently think the URL Standard will end up not normalizing it, but who knows what the future brings so might be best to leave this example out.

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Hmm OK. Are there other examples of things that most servers interpret as equivalent but don't involve fragments (or queries)? I thought that was a nice point made in #443 (comment)

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This is the only one for paths I think.

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hosts works fine too :). But I guess I can find many examples of host canonicalization on my own.

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Hosts normalize during parsing. Before we normalized IP addresses during parsing there was that ability, but no longer.


<p>That is, URL <span data-x="concept-url-query">queries</span> and <span
data-x="concept-url-fragment">fragments</span> can be varied to create distinct entries in the
<span>module map</span>; they are not ignored. Thus, three separate fetches and three separate
module evaluations will be performed.</p>

<p>In contrast, the following code would only create a single entry in the <span>module
map</span>, since after applying the <span>URL parser</span> to these inputs, the resulting <span
data-x="URL record">URL records</span> are equal:</p>

<pre>import "https://example.com/module2.js";
import "https:example.com/module2.js";
import "https://///example.com\\module2.js";
import "https://example.com/foo/../module2.js";</pre>

<p>So in this second example, only one fetch and one module evaluation will occur.</p>

<p>Note that this behavior is the same as how <span
data-x="SharedWorker">shared workers</span> are keyed by their parsed <span
data-x="concept-SharedWorkerGlobalScope-constructor-url">constructor url</span>.</p>
</div>

<p>To <dfn>resolve a module specifier</dfn> given a <span>module script</span> <var>script</var>
and a string <var>specifier</var>, perform the following steps. It will return either an
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