I'm basically reopenning
#10642 since the justification provided for the expected behavior doesn't apply in my case.

Here's a playground, be sure to turn on strictNullChecks:

https://www.typescriptlang.org/play/index.html#src=const%20map%20=%20new%20Map%3Cstring,%20%7Battr:%20number%7D%3E();%0Alet%20obj%20=%20map.get('key');%0Aif%20(!obj)%20%7B%0A%20%20obj%20=%20%7B%20attr:%201%20%7D;%0A%7D%0A[].map(elem%20=%3E%20%7B%0A%20%20console.log(obj.attr);%0A%7D)

TypeScript Version: 3.3.0-dev.201xxxxx

Search Terms:

• strictNullChecks
• control-flow
• undefined
• higher-order function (map, filter, forEach)
• "Object is possibly 'undefined'"

Code

const map = new Map<string, {attr: number}>(); let obj = map.get('key'); if (!obj) { obj = { attr: 1 }; } [].map(elem => { console.log(obj.attr); // error occurs on this line })

Expected behavior:

TypeScript should detect that there's no way obj can become undefined during the evaluation of the map call.