Using optional chaining operator for object property access

问题

TypeScript 3.7 now supports the optional chaining operator. Hence, you can write code such as:

const value = a?.b?.c;

I.e., you can use this operator to access properties of an object, where the object itself may be null or undefined. Now what I would like to do is basically the same, but the property names are dynamic:

const value = a?[b]?.c;

However, there I get a syntax error:

error TS1005: ':' expected.

What am I doing wrong here? Is this even possible?

PS: The proposal seems to imply that this is not possible 😕 (but maybe I get the syntax examples wrong).

回答1:

When accessing a property using bracket notation and optional chaining, you need to also have a dot in addition to the brackets:

const value = a?.[b]?.c;

This is the syntax that was adopted by the TC39 proposal, because otherwise it's hard for the parser to figure out if this ? is part of a ternary expression or part of optional chaining.

回答2:

The Optional Chaining operator is ?.

Here are some examples for nullable property and function handling.

const example = {a: ["first", {b:3}, false]}

// Properties
example?.a  // ["first", {b:3}, false]
example?.b  // undefined

// Dynamic properties ?.[]
example?.a?.[0]     // "first"
example?.a?.[1]?.a  // undefined
example?.a?.[1]?.b  // 3

// Functions ?.()
null?.()                // undefined
validFunction?.()       // result
(() => {return 1})?.()  // 1

Bonus: Default values

?? (Nullish Coalescing) can be used to set a default value if undefined or null.

const notNull = possiblyNull ?? defaultValue
const alsoNotNull = a?.b?.c ?? possiblyNullFallback ?? defaultValue

来源：https://stackoverflow.com/questions/58780817/using-optional-chaining-operator-for-object-property-access