Why does {{#in-element}} only allow null/undefined for insertBefore

davidtaylorhq · April 11, 2024, 4:36pm

The docs for {{#in-element}} say:

insertBefore – by default the DOM element’s content is replaced when used as destinationElement . Passing null to insertBefore changes the behaviour to append the block content to the end of any existing content. Any other value than null is currently not supported.

Being able to pass an element reference to insertBefore would be very useful for my use case. So I was trying to work out why this restriction exists.

If I comment out this assertion in the template compiler:

github.com

emberjs/ember.js/blob/0b79740308/packages/ember-template-compiler/lib/plugins/transform-in-element.ts#L45-L52


      
          if (pair.key === 'insertBefore') {
            assert(
              `Can only pass null to insertBefore in in-element, received: ${JSON.stringify(
                pair.value
              )}`,
              pair.value.type === 'NullLiteral' || pair.value.type === 'UndefinedLiteral'
            );
          }

Then it seems to ‘just work’. I can pass an element to insertBefore, and {{#in-element will render its content before that element. Perfect!

But… I assume there must be a reason this restriction was introduced. Does anyone know why? I can’t see any mention of it in RFC287 which promoted in-element to public API

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Why does {{#in-element}} only allow null/undefined for insertBefore

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