TypeScript - Type 'number | undefined' is not assignable to type 'number'

Question

Here is my code:

var a:number;
a = 4;

var myArr: number[];
myArr = [1,2,3];
myArr.push(1);

a = myArr.pop();

When I compile with "module" (in my tsconfig.json file) set to "system" or "amd" to allow me to bundle the output into an "outFile" location, I get this error:

hello-world.ts:23:1 - error TS2322: Type 'number | undefined' is not assignable to type 'number'. Type 'undefined' is not assignable to type 'number'.

a = myArr.pop();

Where is it getting the "undefined" type from? Also, how do I resolve this error without setting "strict" to false (in my tsconfig.json)?

.pop() returns undefined if the array is empty.

jmoerdyk
– jmoerdyk

2019-06-17 21:14:13 +00:00
Commented Jun 17, 2019 at 21:14 — jmoerdyk
– jmoerdyk, Commented Jun 17, 2019 at 21:14

gatsbyz · Accepted Answer · 2019-06-17 21:14:38Z

9

Array.prototype.pop() returns type number or undefined. That's where number undefined comes from.

When you perform pop() on an object, a number is returned when array.length > 0, and undefined is returned if array.length = 0.

You should either check if myArr.pop() !== undefined

-or-

a: number | undefined;

answered Jun 17, 2019 at 21:14

gatsbyz

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3 Comments

jayarjo Over a year ago

I have !== undefined check in place and still get that error :/

Nyxynyx Over a year ago

Any solutions to the problem raised by @jayarjo?

gatsbyz Over a year ago

@Nyxynyx a = myArr.pop() as number;

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