vscode-fson gives .fson files full syntax highlighting, editing support, and a formatter that matches what the fson library itself produces.

Install

Marketplace (publisher Fedem): search FSON in the Extensions view, or

code --install-extension Fedem.vscode-fson

From a .vsix (on the Downloads page): see Installing a .vsix.

Highlighting

A TextMate grammar (scope source.fson) covering every FSON token:

  • // line and nested /* … */ block comments
  • bare and quoted keys
  • single-, double-, and |-block strings, with escape sequences
  • numbers — hex (0xFF), Infinity / NaN, leading/trailing-dot forms
  • true / false / null
  • %include … as directives
  • -- disabled members, scoped as markup.disabled.fson so they can be dimmed. The extension ships a default rule (#acdea9, italic); override it in your own editor.tokenColorCustomizations if you prefer.

Run Developer: Inspect Editor Tokens and Scopes to see the scope under the cursor.

Editing support

  • line/block comment toggling
  • bracket matching and auto-closing for {}, [], "", ''
  • a wordPattern that matches FSON's narrow bare-key grammar

Formatter

The extension registers a document formatter for .fson — use Format Document or format-on-save. It is a faithful JavaScript port of the fson library's canonical writer, so its output matches Fson::save's: 2-space indent, one member per line, comments (all positions, nested blocks), -- members, %include directives and |-blocks all preserved, number lexemes exact, output a fixed point.

If the document has parse errors the formatter declines to run (so a half-typed file is never mangled) and shows a warning.

Setting — fson.format.preferBareKeys (default false): when on, quoted keys are rewritten bare wherever the name grammar allows. Matches the library's preferBareKeys / fson-check --bare.

Notes

  • A |-block is highlighted as one string; Markdown embedded inside it is intentionally not separately highlighted.
  • The bare-key grammar follows FSON's name production, not JSON5's IdentifierName — $foo, _x, name_, a__b must be quoted.