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ECMAScript proposal for Optional Chaining (aka Existential Operator, aka Null Propagation)
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README.md

Status of this repository

This repository represents the sole opinion of its author. The author’s main goal is that the material presented here could be used as a sound basis for the official ECMAScript proposal.

This repository is now obsolete. The official ECMAScript proposal is here: https://github.com/tc39/proposal-optional-chaining

Optional Chaining

This is a proposal for introducing Optional Chaining feature (aka Existential Operator, aka Null Propagation) in ECMAScript).

Motivation

When looking for a property value deeply in a tree structure, one has often to check whether intermediate nodes exist:

var street = user.address && user.address.street

Also, many API return either an object or null/undefined, and one may want to extract a property from the result only when it is not null:

var fooInput = myForm.querySelector('input[name=foo]')
var fooValue = fooInput ? fooInput.value : undefined

The Optional Chaining Operator allows to handle many of those cases without repeating yourself and/or assigning intermediate results in temporary variables:

var street = user.address?.street

var fooValue = myForm.querySelector('input[name=foo]')?.value

Syntax

The operator is spelt ?. and may be used at the following positions:

obj?.prop         // optional property access
obj?.[expr]       // ditto
func?.(...args)   // optional function or method call
new C?.(...args)  // optional constructor invocation. (But is that case useful?)

Notes

  • In order to allow foo?.3:0 to be parsed as foo ? .3 : 0 (as required for backward compatibility), a simple lookahead is added at the level of the lexical grammar, so that the sequence of characters ?. is not interpreted as a single token in that situation (the ?. token must not be immediately followed by a decimal digit).

  • We don’t use the obj?[expr] and func?(...arg) syntax, because of the difficulty for the parser to distinguish those forms from the conditional operator, e.g. obj?[expr].filter(fun):0 and func?(x - 2) + 3 :1.

    Alternative syntaxes for those two cases have each their own flaws, and deciding which is one the least bad is mostly a question of personal preference. Here is how we made our choice:

    • pick the best syntax for the a?.b case, which is expected to occurs most often;  
    • extend the use of the ?. sequence of characters to other cases, in order to have a uniform look: a?.[b], a?.(b).

Semantics

(The explanations here are optimised for the human mind. For a more machine-friendly version, look at the spec text.)

Base case. If the expression at the left-hand side of the ?. operator evaluates to undefined or null, its right-hand side is not evaluated and the whole expression returns undefined.

a?.b      // undefined if a is null/undefined, a.b otherwise
a?.[++x]   // If a evaluates to null/undefined, the variable x is *not* incremented.

Short-circuiting. A value of undefined produced by the ?. operator is propagated without further evaluation to an entire chain of property accesses, method calls, constructor invocations, etc. (or, in spec parlance, a Left-Hand-Side Expression).

a?.b.c().d      // undefined if a is null/undefined, a.b.c().d otherwise.
                // NB: If a is not null/undefined, and a.b is nevertheless undefined,
                //     short-circuiting does *not* apply

Locality. Apart from short-circuiting, the semantics is strictly local. For instance, the meaning of a . token is not modified by a previous ?. token found earlier in the chain.

a?.b.c().d      // If  a  is not null/undefined, and  a.b  is nevertheless undefined,
                // short-circuiting does *not* apply: the meaning of  .c  is not modified.

Short-circuiting semantics may be compared to an early return instruction in a function.

Free grouping? As currently specced, use of parentheses for mere grouping does not stop short-circuiting. However that semantics is debatable and may be changed.

(a?.b).c().d     // equivalent to: a?.b.c().d

Use in write context. In absence of clear use cases and semantics, the ?. operator is statically forbidden at the left of an assignment operator. On the other hand, optional deletion is allowed, because it has clear semantics, has known use case, and is consistent with the general ”just ignore nonsensical argument” semantics of the delete operator.

a?.b = 42     // trigger an early ReferenceError (same error as `a.b() = c`, etc.)
delete a?.b   // no-op if a is null/undefined

Specification

Technically the semantics are enforced by introducing a special Reference, called Nil, which is propagated without further evaluation through left-hand side expressions (property accesses, method calls, etc.), and which dereferences to undefined.

See the spec text for more details.

Compiling

The Nil Reference is a spec artefact that is used because it is more pleasant to write the spec that way. But if you intend to transform code using optional chaining into es6-compatible code, it is more useful to have an “abrupt completion” mental model. Concretely, the expression:

a?.b.c?.d

could be rewritten using an IIAFE and early return statements:

(() => {
    let _ = a
    if (_ == null)
        return
    _ = _.b.c
    if (_ == null)
        return
    return _.d
})()

For an existing Babel implementation, see: https://github.com/babel/babel/pull/5813

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