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TypeScript - The left-hand side of an assignment expression may not be an optional property access

It has been a while since the last time I did some front end development. So my TypeScript skills are kinda rusty.

Today I tried to write the following code. In this code I'm subscribing to a Server-Send-Event and try to update an element on the screen(I'm not using Angular or any other framework for this small widget).

The TypeScript compiler didn't like this and returned the following error:

The left-hand side of an assignment expression may not be an optional property access

The reason is that I try to use the optional chaining (?.) operator on the left-hand side of an assignment. To solve the error, I have to use an if statement that serves as a type guard instead before the assignment:

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