ECMAScript proposal at stage 4 of the process.
When looking for a property value that's deep in a tree-like structure, one often has 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 : undefinedThe Optional Chaining Operator allows a developer to handle many of those cases without repeating themselves and/or assigning intermediate results in temporary variables:
var street = user.address?.street
var fooValue = myForm.querySelector('input[name=foo]')?.valueWhen some other value than undefined is desired for the missing case, this can usually be handled with the Nullish coalescing operator:
// falls back to a default value when response.settings is missing or nullish
// (response.settings == null) or when response.settings.animationDuration is missing
// or nullish (response.settings.animationDuration == null)
const animationDuration = response.settings?.animationDuration ?? 300;
The call variant of Optional Chaining is useful for dealing with interfaces that have optional methods:
iterator.return?.() // manually close an iteratoror with methods not universally implemented:
if (myForm.checkValidity?.() === false) { // skip the test in older web browsers
// form validation fails
return;
}Unless otherwise noted, in the following languages, the syntax consists of a question mark prepending the operator, (a?.b, a?.b(), a?[b] or a?(b) when applicable).
The following languages implement the operator with the same general semantics as this proposal (i.e., 1) guarding against a null base value, and 2) short-circuiting application to the whole chain):
new a?()). Also applies to assignment and deletion.The following languages have a similar feature, but do not short-circuit the whole chain when it is longer than one element. This is justified by the fact that, in those languages, methods or properties might be legitimately used on null (e.g., null.toString() == "null" in Dart):
a&.bThe following languages have a similar feature. We haven’t checked whether they have significant differences in semantics with this proposal:
The Optional Chaining operator is spelled ?.. It may appear in three positions:
obj?.prop // optional static property access
obj?.[expr] // optional dynamic property access
func?.(...args) // optional function or method callfoo?.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).If the operand at the left-hand side of the ?. operator evaluates to undefined or null, the expression evaluates to undefined. Otherwise the targeted property access, method or function call is triggered normally.
Here are basic examples, each one followed by its desugaring. (The desugaring is not exact in the sense that the LHS should be evaluated only once and that document.all should behave as an object.)
a?.b // undefined if `a` is null/undefined, `a.b` otherwise.
a == null ? undefined : a.b
a?.[x] // undefined if `a` is null/undefined, `a[x]` otherwise.
a == null ? undefined : a[x]
a?.b() // undefined if `a` is null/undefined
a == null ? undefined : a.b() // throws a TypeError if `a.b` is not a function
// otherwise, evaluates to `a.b()`
a?.() // undefined if `a` is null/undefined
a == null ? undefined : a() // throws a TypeError if `a` is neither null/undefined, nor a function
// invokes the function `a` otherwiseIf the expression on the LHS of ?. evaluates to null/undefined, the RHS is not evaluated. This concept is called short-circuiting.
a?.[++x] // `x` is incremented if and only if `a` is not null/undefined
a == null ? undefined : a[++x]In fact, short-circuiting, when triggered, skips not only the current property access, method or function call, but also the whole chain of property accesses, method or function calls directly following the Optional Chaining operator.
a?.b.c(++x).d // if `a` is null/undefined, evaluates to undefined. Variable `x` is not incremented.
// otherwise, evaluates to `a.b.c(++x).d`.
a == null ? undefined : a.b.c(++x).dNote that the check for nullity is made on a only. If, for example, a is not null, but a.b is null, a TypeError will be thrown when attempting to access the property "c" of a.b.
This feature is implemented by, e.g., C# and CoffeeScript; see Prior Art.
Let’s call Optional Chain an Optional Chaining operator followed by a chain of property accesses, method or function calls.
An Optional Chain may be followed by another Optional Chain.
a?.b[3].c?.(x).d
a == null ? undefined : a.b[3].c == null ? undefined : a.b[3].c(x).d
// (as always, except that `a` and `a.b[3].c` are evaluated only once)Parentheses limit the scope of short-circuiting:
(a?.b).c
(a == null ? undefined : a.b).cThat follows from the design choice of specifying the scope of short-circuiting by syntax (like the && operator), rather than propagation of a Completion (like the break instruction) or an adhoc Reference (like an earlier version of this proposal). In general, syntax cannot be arbitrarily split by parentheses: for example, ({x}) = y is not destructuring assignment, but an attempt to assign a value to an object literal.
Note that, whatever the semantics are, there is no practical reason to use parentheses in that position anyway.
Because the delete operator is very liberal in what it accepts, we have that feature for free:
delete a?.b
a == null ? true : delete a.bwhere true is the usual result of attempting to delete a non-Reference.
Although they could be included for completeness, the following are not supported due to lack of real-world use cases or other compelling reasons; see Issue # 22 and Issue #54 for discussion:
new a?.()a?.`string`new a?.b(), a?.b`string`The following is not supported, although it has some use cases; see Issue #18 for discussion:
a?.b = cThe following are not supported, as it does not make much sense, at least in practice; see Issue #4 (comment):
super?.(), super?.foonew?.target, import?.('foo'), etc.All the above cases will be forbidden by the grammar or by static semantics so that support might be added later.
There has been various interesting ideas for applying the idea of “optional” to other constructs. However, there are not part of this proposal. For example:
Issue #28: Should optional chains support the upcoming private class fields and private methods, as in a?.#b, a?.#b() or a?.b.#c? Quoting microsoft/TypeScript#30167 (comment):
This one isn't baked into the proposal yet, simply because private fields themselves aren't baked yet. So we don't want to hold up this proposal if that one happens to stall out. Once that one has reached Stage 4, we will address it then.
obj?.[expr] and func?.(arg) look ugly. Why not use obj?[expr] and func?(arg) as does <language X>?(null)?.b evaluate to undefined rather than null?foo?.() throw when foo is neither nullish nor callable?a?.b.c, if a.b is null, then a.b.c will evaluate to undefined, right?a?.b?.c, why should I write ?. at each level? Should I not be able to write the operator only once for the whole chain?null/undefined at each level, no?See: https://tc39.github.io/proposal-optional-chaining/