Typescript为什么类型防护不能对可为null的联合类型的对象的数组按预期方式工作（启用strictNullChecks）

Question

type Field = {test: {more: number} | null}
let fields: Field[] = [{test: {more: 55}}]

Transpiler throws error regardless of the type guard:

if (fields[0].test) {
  fields[0].test.more = 55 // object is possibly null
} 

Here no error:

function f(field: Field) {
  if (field.test) field.test.more = 15 // no error
}

Answer 1

Type flow does not keep track of array index access, so it will not remember that you checked the 0 index for null . This was considered but apparently not implemented due to performance considerations You can put the value in a local variable and type guards will work as expected on those:

type Field = {test: {more: number} | null}
let fields: Field[] = [{test: {more: 55}}]
let d = fields[0];
if (d.test) {
    d.test.more = 55 // object is possibly null
} 

Answer 2

Well, it seems that TypeScript doesn't keep track of nulls in Array indexes, which is a bit strange ... maybe an issue could be opened at GitHub about it. Anyway, there is a way to prevent that error. Once you make that check, you know that test is not null , so you can do:

if (fields[0].test !== null) {
  fields[0].test!.more = 55 // object is possibly null
}

The ! after test will tell the compiler that that variable is defined and different from null or undefined .