Object is possibly 'undefined'

[troy351]

TypeScript Version: 3.4.0-dev.201xxxxx

Search Terms: Object undefined

Code turn strictNullChecks on

// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.
type EventType = 'click' | 'dblclick'

const handlerMap: { [P in EventType]?: any[] } = {}

function addHandler<P extends EventType>(evType: P) {
  const handlerList = handlerMap[evType] || []
  handlerList.push({}) // Error here: Object is possibly 'undefined'
  handlerMap[evType] = handlerList
}

Expected behavior: No Error

Actual behavior: Shows Error

Playground Link: http://www.typescriptlang.org/play/#src=type%20EventType%20%3D%20'click'%20%7C%20'dblclick'%0Aconst%20handlerMap%3A%20%7B%20%5BP%20in%20EventType%5D%3F%3A%20any%5B%5D%20%7D%20%3D%20%7B%7D%0Afunction%20addHandler%3CP%20extends%20EventType%3E(evType%3A%20P)%20%7B%0A%20%20const%20handlerList%20%3D%20handlerMap%5BevType%5D%20%7C%7C%20%5B%5D%0A%20%20handlerList.push(%7B%7D)%20%2F%2F%20Error%20here%3A%20Object%20is%20possibly%20'undefined'%0A%7D%0A

Related Issues:

[collin5]

Will be giving this a try.

[ktoto]

It's the same issue?

interface State {
    [i: string]: {data?: {id:string}} | undefined
 }

declare var x: State;

const id = 'xx';
const y = x[id] && x[id].data; // Error: Object is possibly 'undefined' (with strictNullChecks
const yId = y && y.id; // correct type – string | undefined

Playground link : https://www.typescriptlang.org/play/#src=interface%20State%20%7B%0D%0A%20%20%20%20%5Bi%3A%20string%5D%3A%20%7Bdata%3F%3A%20%7Bid%3Astring%7D%7D%20%7C%20undefined%0D%0A%20%7D%0D%0A%0D%0Adeclare%20var%20x%3A%20State%3B%0D%0A%0D%0Aconst%20id%20%3D%20'xx'%3B%0D%0A%0D%0Aconst%20y%20%3D%20x%5Bid%5D%20%26%26%20x%5Bid%5D.data%3B%0D%0A%0D%0Aconst%20yId%20%3D%20y%20%26%26%20y.id%3B%0D%0A

[weswigham]

#29317 should be the fix here (which I have mentioned in @collin5 's PR). Specifically, the type of a || b should be calculated as (typeof a & not FalseyTypes) | typeof b, where FalseyTypes is 0 | false | "" | undefined | null.

@RyanCavanaugh given that, should we move this out of 3.4, since we won't be shipping them in 3.4?

[weswigham]

@ktoto your issue is different - since State has an index signature, we don't track refinements on individual property lookups within it - see #17960 and #29042 (and go updoot those issues so they don't sit in the backlog).

[MufidJamaluddin]

I have same issue. :(

Typescript Version : 3.4.5

[troy351]

@MufidJamaluddin duplicate of #7719

Workaround is use kelas!.iskelas

[micky2be]

Same issue as @MufidJamaluddin
I have a checker in place to avoid duplicating code but Typescript ignores it and output this error.