The purpose of this document is to collect invariants that have existed in the past, but been dropped for one reason or another. We may not remember why, so this field is optional. Please add invariants as you see fit using the template.

Abandoned Invariants

Invariants that were broken

Script Concatination

Description

Script Concatination can always have a comment added to it at the start without effect.

Specification Details

Was contested in relation to hashbang grammer, but was not upheld.

Rationale

Insertion of license comments does not break.

Impact of breakage

requires more complex parsing.

Documentation

See hashbang notes (TODO find links)

Top lexical scope constructs OR Everything is generative

Description

Prior to modules, any expression could be nested in a function and it would be created each time the function was called. Allowed you to take anything and put it into an IIFE and it would work. Anything that you can create, you can multiply instantiate.

Breakage

Since import / exports syntax were introduced, this was the first syntax that only appears at top level. it was the first syntax that could not be in the body of the function.

Specification Details

Import / Export .

Rationale

TODO.

Documentation

(TODO find links)

InstanceOf operator

Description

instanceOf operator used to be a reliable way to .

Breakage

Introduction of Symbol.hasInstance.

Specification Details

See introduction of Symbols.

Rationale

TODO

Documentation

(TODO find links)

Object.prototype.ToString.call as a brand

Description

toString used to be a brand check. That Symbol.toStringTag would remove this invariant was discussed immediately prior to ES6, and since all builtin types (or so we thought, at the time) had a prototype method that brand-checked and threw an exception, this was deemed sufficient for all future types - in other words, a new invariant was added at that time, that every builtin type must have a means by which it can be brand-checked, even if indirectly. The Error types remain the only (overlooked at the time) builtin types for which this does not hold.

Breakage

Introduction of Symbol.toStringTag.

Specification Details

TODO

Rationale

It was initially an accident, but it was useful as a brand check.

Documentation

TODO

Setters and Getters / Accessors

Description

Spec invariant

Breakage

All of the browsers had accessors, __defineGetter__ __defineSetter__.

Specification Details

ES5 rule that anything that is true in 3 out of the 5 browsers is a strong candidate for specification.

Rationale

TODO

Documentation

TODO

Potentially Abandoned Invariants

Invariants that may be broken soon

typeof and the behavior of == and ===

Description

If the result of typeof a and b were the same, the result from == and === should be the same. This was relaxed in the cases of anticipated value types and operator overloading.

Specification Details

Was important during ES5 discussions

Rationale

TODO This is a guess -- Consistency.

Documentation

TODO Find documentation

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Template

## Title

### Description

### Specification Details

### Rationale

### Documentation