Optional chaining (?.)

optional chaining operator ( ?. ) permits reading the value of a property located deep within a chain of connected objects without having to expressly validate that each reference in the chain is valid. ?. operator functions similarly to the . chaining operator, except that instead of causing an error if a reference is nullish ( null or undefined ), the expression short-circuits with a return value of undefined . When used with function calls, it returns undefined if the given function does not exist.

This results in shorter and simpler expressions when accessing chained properties when the possibility exists that a reference may be missing. It can also be helpful while exploring the content of an object when there's no known guarantee as to which properties are required.

句法

obj?.prop
obj?.[expr]
arr?.[index]
func?.(args)
					

描述

The optional chaining operator provides a way to simplify accessing values through connected objects when it's possible that a reference or function may be undefined or null .

For example, consider an object obj which has a nested structure. Without optional chaining, looking up a deeply-nested subproperty requires validating the references in between, such as:

let nestedProp = obj.first && obj.first.second;
					

The value of obj.first is confirmed to be non- null (and non- undefined ) before then accessing the value of obj.first.second . This prevents the error that would occur if you simply accessed obj.first.second directly without testing obj.first .

With the optional chaining operator ( ?. ), however, you don't have to explicitly test and short-circuit based on the state of obj.first before trying to access obj.first.second :

let nestedProp = obj.first?.second;
					

By using the ?. operator instead of just . , JavaScript knows to implicitly check to be sure obj.first 不是 null or undefined before attempting to access obj.first.second 。若 obj.first is null or undefined , the expression automatically short-circuits, returning undefined .

This is equivalent to the following, except that the temporary variable is in fact not created:

let temp = obj.first;
let nestedProp = ((temp === null || temp === undefined) ? undefined : temp.second);
					

Optional chaining with function calls

You can use optional chaining when attempting to call a method which may not exist. This can be helpful, for example, when using an API in which a method might be unavailable, either due to the age of the implementation or because of a feature which isn't available on the user's device.

Using optional chaining with function calls causes the expression to automatically return undefined instead of throwing an exception if the method isn't found:

let result = someInterface.customMethod?.();
					

注意: If there is a property with such a name and which is not a function, using ?. will still raise a TypeError exception ( someInterface.customMethod is not a function ).

注意: someInterface itself is null or undefined TypeError exception will still be raised ( someInterface is null ). If you expect that someInterface itself may be null or undefined , you have to use ?. at this position as well: someInterface?.customMethod?.()

Dealing with optional callbacks or event handlers

If you use callbacks or fetch methods from an object with a destructuring assignment , you may have non-existent values that you cannot call as functions unless you have tested their existence. Using ?. , you can avoid this extra test:

// Written as of ES2019
function doSomething(onContent, onError) {
  try {
    // ... do something with the data
  }
  catch (err) {
    if (onError) { // Testing if onError really exists
      onError(err.message);
    }
  }
}
					
// Using optional chaining with function calls
function doSomething(onContent, onError) {
  try {
   // ... do something with the data
  }
  catch (err) {
    onError?.(err.message); // no exception if onError is undefined
  }
}
					

Optional chaining with expressions

You can also use the optional chaining operator when accessing properties with an expression using the bracket notation of the property accessor :

let nestedProp = obj?.['prop' + 'Name'];
					

Optional chaining not valid on the left-hand side of an assignment

let object = {};
object?.property = 1; // Uncaught SyntaxError: Invalid left-hand side in assignment
					

Array item access with optional chaining

let arrayItem = arr?.[42];
					

范例

Basic example

This example looks for the value of the name property for the member bar in a map when there is no such member. The result is therefore undefined .

let myMap = new Map();
myMap.set("foo", {name: "baz", desc: "inga"});
let nameBar = myMap.get("bar")?.name;
					

Short-circuiting evaluation

When using optional chaining with expressions, if the left operand is null or undefined , the expression will not be evaluated. For instance:

let potentiallyNullObj = null;
let x = 0;
let prop = potentiallyNullObj?.[x++];
console.log(x); // 0 as x was not incremented
					

Stacking the optional chaining operator

With nested structures, it is possible to use optional chaining multiple times:

let customer = {
  name: "Carl",
  details: {
    age: 82,
    location: "Paradise Falls" // detailed address is unknown
  }
};
let customerCity = customer.details?.address?.city;
// … this also works with optional chaining function call
let duration = vacations.trip?.getTime?.();
					

Combining with the nullish coalescing operator

nullish coalescing operator may be used after optional chaining in order to build a default value when none was found:

let customer = {
  name: "Carl",
  details: { age: 82 }
};
const customerCity = customer?.city ?? "Unknown city";
console.log(customerCity); // Unknown city
					

规范

规范
ECMAScript (ECMA-262)
The definition of 'optional expression' in that specification.

浏览器兼容性

更新 GitHub 上的兼容性数据
Desktop Mobile Server
Chrome Edge Firefox Internet Explorer Opera Safari Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet Node.js
Optional chaining operator ( ?. ) Chrome 80
80
79
Disabled
Disabled From version 79: this feature is behind the Experimental JavaScript preference (needs to be set to true ). To change preferences in Chrome, visit chrome://flags.
Edge 80
80
79
Disabled
Disabled From version 79: this feature is behind the Experimental JavaScript preference (needs to be set to true ).
Firefox 74 IE No Opera 67
67
66
Disabled
Disabled From version 66: this feature is behind the Experimental JavaScript preference (needs to be set to true ).
Safari 13.1 WebView Android 80 Chrome Android 80
80
79
Disabled
Disabled From version 79: this feature is behind the Experimental JavaScript preference (needs to be set to true ). To change preferences in Chrome, visit chrome://flags.
Firefox Android No Opera Android No Safari iOS 13.4 Samsung Internet Android No nodejs 14.0.0

图例

完整支持
完整支持
不支持
不支持
用户必须明确启用此特征。
用户必须明确启用此特征。

实现进度

The following table provides a daily implementation status for this feature because this feature has not yet reached cross-browser stability. The data is generated by running the relevant feature tests in Test262 , the standard test suite of JavaScript, in the nightly build, or the latest release of each browser's JavaScript engine.

另请参阅

Metadata

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    31. Logical AND assignment (&&=)
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    33. Logical OR (||)
    34. Logical OR assignment (||=)
    35. Logical nullish assignment (??=)
    36. Multiplication (*)
    37. Multiplication assignment (*=)
    38. Nullish coalescing operator (??)
    39. Object initializer
    40. 运算符优先级
    41. Optional chaining (?.)
    42. Pipeline operator (|>)
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    22. SyntaxError: Using //@ to indicate sourceURL pragmas is deprecated. Use //# instead
    23. SyntaxError: a declaration in the head of a for-of loop can't have an initializer
    24. SyntaxError: applying the 'delete' operator to an unqualified name is deprecated
    25. SyntaxError: for-in loop head declarations may not have initializers
    26. SyntaxError: function statement requires a name
    27. SyntaxError: identifier starts immediately after numeric literal
    28. SyntaxError: illegal character
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    30. SyntaxError: missing ) after argument list
    31. SyntaxError: missing ) after condition
    32. SyntaxError: missing : after property id
    33. SyntaxError: missing ; before statement
    34. SyntaxError: missing = in const declaration
    35. SyntaxError: missing ] after element list
    36. SyntaxError: missing formal parameter
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    38. SyntaxError: missing variable name
    39. SyntaxError: missing } after function body
    40. SyntaxError: missing } after property list
    41. SyntaxError: redeclaration of formal parameter "x"
    42. SyntaxError: return not in function
    43. SyntaxError: test for equality (==) mistyped as assignment (=)?
    44. SyntaxError: unterminated string literal
    45. TypeError: "x" has no properties
    46. TypeError: "x" is (not) "y"
    47. TypeError: "x" is not a constructor
    48. TypeError: "x" is not a function
    49. TypeError: "x" is not a non-null object
    50. TypeError: "x" is read-only
    51. TypeError: 'x' is not iterable
    52. TypeError: More arguments needed
    53. TypeError: Reduce of empty array with no initial value
    54. TypeError: X.prototype.y called on incompatible type
    55. TypeError: can't access dead object
    56. TypeError: can't access property "x" of "y"
    57. TypeError: can't assign to property "x" on "y": not an object
    58. TypeError: can't define property "x": "obj" is not extensible
    59. TypeError: can't delete non-configurable array element
    60. TypeError: can't redefine non-configurable property "x"
    61. TypeError: cannot use 'in' operator to search for 'x' in 'y'
    62. TypeError: cyclic object value
    63. TypeError: invalid 'instanceof' operand 'x'
    64. TypeError: invalid Array.prototype.sort argument
    65. TypeError: invalid arguments
    66. TypeError: invalid assignment to const "x"
    67. TypeError: property "x" is non-configurable and can't be deleted
    68. TypeError: setting getter-only property "x"
    69. TypeError: variable "x" redeclares argument
    70. URIError: malformed URI sequence
    71. Warning: -file- is being assigned a //# sourceMappingURL, but already has one
    72. Warning: 08/09 is not a legal ECMA-262 octal constant
    73. Warning: Date.prototype.toLocaleFormat is deprecated
    74. Warning: JavaScript 1.6's for-each-in loops are deprecated
    75. Warning: String.x is deprecated; use String.prototype.x instead
    76. Warning: expression closures are deprecated
    77. Warning: unreachable code after return statement
  13. 杂项
    1. JavaScript technologies overview
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    3. JavaScript 数据结构
    4. Enumerability and ownership of properties
    5. Iteration protocols
    6. 严格模式
    7. Transitioning to strict mode
    8. Template literals
    9. 弃用特征