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Fix ScriptTagHelper regression by checking for existing content before processing importmaps #63155

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Fix ScriptTagHelper regression by checking for existing content before processing importmaps #63155

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Copilot Aug 6, 2025
9a91938
Fix script tag importmap regression - preserve user content when no a…
Copilot Aug 6, 2025
15776b3
Fix ScriptTagHelper importmap regression by checking for existing con…
Copilot Aug 11, 2025
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25 changes: 17 additions & 8 deletions src/Mvc/Mvc.TagHelpers/src/ScriptTagHelper.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -247,16 +247,25 @@ public override void Process(TagHelperContext context, TagHelperOutput output)
var importMap = ImportMap ?? ViewContext.HttpContext.GetEndpoint()?.Metadata.GetMetadata<ImportMapDefinition>();
if (importMap == null)
{
// No importmap found, nothing to do.
output.SuppressOutput();
// No importmap found. Only suppress output if this was intended to be
// an automatically generated importmap (i.e., when asp-importmap was used).
// If the user provided explicit content without asp-importmap, let it render as-is.
if (ImportMap != null || context.AllAttributes.ContainsName(ImportMapAttributeName))
{
output.SuppressOutput();
return;
}
// Let the tag render as-is by continuing with normal processing
// Don't return here, let normal attribute copying happen
}
else
{
output.TagName = "script";
output.TagMode = TagMode.StartTagAndEndTag;
output.Attributes.SetAttribute("type", "importmap");
output.Content.SetHtmlContent(importMap.ToString());
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I don't think this is the right fix.

If the goal is to do nothing if there is already content on the script, then what should happen is:

  • Call var originalContent = await output.GetChildContentAsync().
  • Check for content.IsEmptyOrWhitespace
  • Set the import map only in that situation.

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Note that we still want to process all <script type="importmap"></script> but the fix is only to avoid replacing existing content if there's already data the developer put it.

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@copilot @MackinnonBuck

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Fixed in commit 15776b3. I've implemented the approach you suggested:

  1. Call await output.GetChildContentAsync() to get existing content
  2. Check if content is empty or whitespace using IsEmptyOrWhiteSpace
  3. If content exists, preserve it and return early
  4. If no content exists, apply the normal importmap logic

The fix now properly processes all <script type="importmap"></script> tags but avoids replacing existing content when developers have provided their own importmap data. I've maintained backward compatibility by keeping the original Process method that calls the new ProcessAsync method.

return;
}

output.TagName = "script";
output.TagMode = TagMode.StartTagAndEndTag;
output.Attributes.SetAttribute("type", "importmap");
output.Content.SetHtmlContent(importMap.ToString());
return;
}

// Pass through attribute that is also a well-known HTML attribute.
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53 changes: 53 additions & 0 deletions src/Mvc/Mvc.TagHelpers/test/ScriptTagHelperTest.cs
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Expand Up @@ -794,6 +794,59 @@ public void ScriptTagHelper_RendersImportMap_FromEndpoint()
Assert.Equal(importMap.ToJson(), output.Content.GetContent());
}

[Fact]
public void ScriptTagHelper_PreservesExplicitImportMapContent_WhenNoImportMapDefinition()
{
// Arrange - this simulates the user's scenario where they provide explicit importmap content
// without using asp-importmap attribute
var context = MakeTagHelperContext(
attributes: new TagHelperAttributeList
{
new TagHelperAttribute("type", "importmap"),
});

var output = MakeTagHelperOutput("script", attributes: new TagHelperAttributeList());

var helper = GetHelper();
helper.Type = "importmap";
// No endpoint with ImportMapDefinition and no asp-importmap attribute
// This should NOT suppress the output, allowing user content to render

// Act
helper.Process(context, output);

// Assert
Assert.Equal("script", output.TagName); // Tag should not be suppressed
Assert.Equal("importmap", output.Attributes["type"].Value);
// The output should not be suppressed, allowing user's explicit content to render
Assert.False(output.IsContentModified); // Content should remain as user provided
}

[Fact]
public void ScriptTagHelper_SuppressesOutput_WhenAspImportMapAttributeUsedButNoDefinition()
{
// Arrange - this simulates using asp-importmap attribute but having no ImportMapDefinition
var context = MakeTagHelperContext(
attributes: new TagHelperAttributeList
{
new TagHelperAttribute("type", "importmap"),
new TagHelperAttribute("asp-importmap", null), // asp-importmap used but no value
});

var output = MakeTagHelperOutput("script", attributes: new TagHelperAttributeList());

var helper = GetHelper();
helper.Type = "importmap";
// No endpoint with ImportMapDefinition but asp-importmap attribute is present
// This should suppress the output since it was intended to be auto-generated

// Act
helper.Process(context, output);

// Assert - output should be suppressed when asp-importmap is used but no definition found
Assert.Null(output.TagName); // Tag should be suppressed
}

private Endpoint CreateEndpoint(ImportMapDefinition importMap = null)
{
return new Endpoint(
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