For AI agents: the complete documentation index is available at /tc39-atlas/en/llms.txt, the full documentation bundle is available at /tc39-atlas/en/llms-full.txt, and this page is available as Markdown at /tc39-atlas/en/proposals/year/pending/proposal-object-freeze-seal-syntax.md.
TC39 Atlas
  • English

      • Object.freeze + Object.seal syntax S1

      Proposal details
      Proposal overview

      This proposal introduces syntax for Object.freeze and Object.seal to improve ergonomics and prevent modification through prototype chains. It sketches syntax like {# . #} for freezing and {| . |} for sealing, applicable to object literals, arrays, destructuring patterns, and function parameters. The proposal is at an early stage with sketches and potential extensions, not yet a formal TC39 proposal.

      Note

      The README below comes from the upstream repository and may contain outdated stage or status metadata. Use the proposal details above as the current source of truth.

      Object.freeze and Object.seal syntax

      Rationale

      Object.freeze and Object.seal are both useful functions, but they aren't particularly ergonomic to use, especially when dealing with deeply nested objects or trying to do complex things like create a superset of a frozen object. Also even frozen objects can be modified via their prototype chain:

      > const x = Object.freeze({});
undefined
> x.foo
undefined
> Object.prototype.foo = 3;
3
> x.foo
3

      To prevent this you can use Object.freeze({ __proto__: null }) or Object.freeze(Object.assign(Object.create(null), {})) to ensure that frozen objects cannot be modified via their prototype chain.

      > const x = Object.freeze({ __proto__: null });
undefined
> x.foo
undefined
> Object.prototype.foo = 3;
3
> x.foo
undefined

      In addition, it would be useful to have these syntaxes in other places. It'd be useful to seal a destructuring expression, or freeze function arguments to create immutable bindings.

      Sketches

      Basic freezing of an object

      const foo = {#
  a: {#
    b: {#
      c: {#
        d: {#
          e: [# "some string!" #]
        #}
      #}
    #}
  #}
#}
      Click here to see the desugared version 
      const foo = Object.freeze({
  __proto__: null,
  a: Object.freeze({
    __proto__: null,
    b: Object.freeze({
      __proto__: null,
      c: Object.freeze({
        __proto__: null,
        d: Object.freeze({
          __proto__: null,
          e: Object.freeze([ "some string!" ])
        })
      })
    })
  })
})

      Basic sealing of an object

      const foo = {|
  a: {|
    b: {|
      c: {|
        d: {|
          e: [| "some string!" |]
        |}
      |}
    |}
  |}
|}
      Click here to see the desugared version 
      const foo = Object.seal({
  __proto__: null,
  a: Object.seal({
    __proto__: null,
    b: Object.seal({
      __proto__: null,
      c: Object.seal({
        __proto__: null,
        d: Object.seal({
          __proto__: null,
          e: Object.seal(["some string!"])
        })
      })
    })
  })
})

      Sealing a functions destructured options bag

      function ajax({| url, headers, onSuccess |}) {
  fetch(url, { headers }).then(onSuccess)
}
ajax({ url: 'http://example.com', onsuccess: console.log })
// throws TypeError('cannot define property `onsuccess`. Object is not extensible')
      Click here to see the desugared version 
      function ajax(_ref1) {
  const _ref2 = Object.seal({ url: undefined, headers: undefined, onSuccess: undefined })
  Object.assign(_ref2, _ref1)
  let url = _ref2.url
  let headers = _ref2.headers
  let onSuccess = _ref2.onSuccess

  fetch(url, { headers }).then(onSuccess)
}
ajax({ url: 'http://example.com', onsuccess: console.log })
// throws TypeError('cannot define property `onsuccess`. Object is not extensible')

      Freezing a functions destructured options bag

      function ajax({# url, headers, onSuccess #}) {
  url = new URL(url) // throws TypeError('cannot assign to const `url`')
  fetch(url, { headers }).then(onSuccess)
}
ajax({ url: 'http://example.com', onSuccess: console.log })
      Click here to see the desugared version 
      function ajax(_ref1) {
  const _ref2 = Object.seal({ url: undefined, headers: undefined, onSuccess: undefined }) // seal now, const later
  Object.assign(_ref2, _ref1)
  const url = _ref2.url
  const headers = _ref2.headers
  const onSuccess = _ref2.onSuccess

  url = new URL(url) // throws TypeError('cannot assign to const `url`')
  fetch(url, { headers }).then(onSuccess)
}
ajax({ url: 'http://example.com', onSuccess: console.log })

      Potential extensions - sketches

      Sealed function param bindings

      function add(| a, b |) {
  return a + b
}
add(2, 2, 2) === 6
// throws TypeError('invalid third parameter, expected 2`)
      Click here to see the desugared version 
      function add(_1, _2) {
  if (arguments.length > 2) {
    throws TypeError('invalid third parameter, expected 2')
  }
  let a = arguments[0]
  let b = arguments[1]

  return a + b
}
add(2, 2, 2) === 6
// throws TypeError('invalid third parameter, expected 2`)

      Frozen function param bindings

      function add1(# a #) {
  a += 1 // throws TypeError `invalid assignment...`
  return a
}
add1(1) === 2
      Click here to see the desugared version 
      function add1(_1) {
  if (arguments.length > 1) {
    throws TypeError('invalid second parameter, expected 1')
  }
  const a = arguments[0]

  a += 1 // throws TypeError `invalid assignment...`
  return a
}
add1(1) === 2

      Extending syntax to general destructing for better type safety

      const foo = { a: 1, b: 2 }
const {| a, b, c |} = foo
// Throws TypeError 'invalid assignment to unknown property c'
      Click here to see the desugared version 
      const foo = { a: 1, b: 2 }
if (!('a' in foo)) throw new TypeError('invalid assignment to unknown property a')
const a = foo.a
if (!('b' in foo)) throw new TypeError('invalid assignment to unknown property b')
const b = foo.b
if (!('c' in foo)) throw new TypeError('invalid assignment to unknown property c')
const c = foo.c
// Throws TypeError 'invalid assignment to unknown property c'