When I do the following in vscode, it is a union of string and null, but for some reason it is only a string.

let foo: null | string;

image

i want to know the cause

sorry for bothering you, but please tell me!!!!

CodePudding user response：

Probably because it is a special case in vscode and is marked as something like a nullable string. If you want to know exactly what happens, you can always read the vscode source code on github.

CodePudding user response：

This is because that in the tsconfig file, strictNullChecks property is false. When it is false it makes all types like number, string, etc, more extended so that null and undefined types will be as a part of that type. So string type (that is shown by vscode) in strictNullChecks = false mode, is actually

string | null | undefined

But if you change strictNullChecks value to true you will see that vscode shows that just like

string | null

If the file you have mentioned is not part of an existing project and there is no tsconfig file that you could find, search for Implicit Project Config: Strict Null Checks in vscode settings and change that property.