Simon Willison’s Weblog

1st June 2025 - Link Blog

Progressive JSON. This post by Dan Abramov is a trap! It proposes a fascinating way of streaming JSON objects to a client in a way that provides the shape of the JSON before the stream has completed, then fills in the gaps as more data arrives... and then turns out to be a sneaky tutorial in how React Server Components work.

Ignoring the sneakiness, the imaginary streaming JSON format it describes is a fascinating thought exercise:

{
  header: "$1",
  post: "$2",
  footer: "$3"
}
/* $1 */
"Welcome to my blog"
/* $3 */
"Hope you like it"
/* $2 */
{
  content: "$4",
  comments: "$5"
}
/* $4 */
"This is my article"
/* $5 */
["$6", "$7", "$8"]
/* $6 */
"This is the first comment"
/* $7 */
"This is the second comment"
/* $8 */
"This is the third comment"

After each block the full JSON document so far can be constructed, and Dan suggests interleaving Promise() objects along the way for placeholders that have not yet been fully resolved - so after receipt of block $3 above (note that the blocks can be served out of order) the document would look like this:

{
  header: "Welcome to my blog",
  post: new Promise(/* ... not yet resolved ... */),
  footer: "Hope you like it"
}

I'm tucking this idea away in case I ever get a chance to try it out in the future.