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Disallow < after a JSX element #141

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Disallow < after a JSX element #141

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3ac6d71
Disallow `<` after a JSX element
nicolo-ribaudo Feb 25, 2022
58edf8d
Update spec.emu
nicolo-ribaudo Mar 5, 2022
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30 changes: 27 additions & 3 deletions spec.emu
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Expand Up @@ -52,17 +52,41 @@ render(dropdown);

<h2>Syntax</h2>
<emu-grammar>
PrimaryExpression :
<ins>JSXElement</ins>
<ins>JSXFragment</ins>
PrimaryExpression ::
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<ins>JSXElementOrFragment `<`</ins>
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Do we need this production to be able to define an early error on PrimaryExpression :: JSXElementOrFragment <?

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We could do something like

PrimaryExpression :: JSXElementOrFragment

- It is a Syntax Error is this production is followed by the `<` token

but I used that new production to avoid introducing new language/checks that are not already used in the ecma262 spec

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@Huxpro Huxpro Feb 28, 2022 •
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I understand that without PrimaryExpression :: JSXElementOrFragment < we don't have a production to match with. But it's also not ideal to me to only add this production specifically for the early error 🤔

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Note that the ecma262 spec already does this in a few cases, for example to disallow a?.b`c`: https://tc39.es/ecma262/#sec-left-hand-side-expressions-static-semantics-early-errors

<ins>JSXElementOrFragment</ins>
</emu-grammar>

<emu-clause type="sdo" id="sec-jsx-PrimaryExpression-early-errors">
<h1>Static Semantics: Early Errors</h1>
<emu-grammar>
PrimaryExpression :: JSXElementOrFragment `<`
</emu-grammar>
<ul>
<li>It is a SyntaxError if any source text is matched by this production.</li>
</ul>
<emu-note>
This Early Error prevents users from accidentally writing two adjacent JSX elements without wrapping them in a fragment:
<pre><code class="language-jsx">
// Invalid
var elts = &lt;a&gt;&lt;/a&gt;&lt;b&gt;&lt;/b&gt;

// Valid
var elts = &lt;&gt;&lt;a&gt;&lt;/a&gt;&lt;b&gt;&lt;/b&gt;&lt;/&gt;
</code></pre>
</emu-note>
</emu-clause>
</emu-clause>

<emu-clause id="sec-jsx-elements">
<h1>JSX Elements</h1>
<h2>Syntax</h2>

<emu-grammar type="definition">
JSXElementOrFragment ::
JSXElement
JSXFragment

JSXElement :
JSXSelfClosingElement
JSXOpeningElement JSXChildren? JSXClosingElement
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