CODEWITHSUNDEEP
typescript
Jez
Jez

Reputation: 29993

Can you use the question mark syntax when declaring a variable?

When creating a TypeScript interface one can make an entry "optional", or type | undefined, by adding a question mark, eg.

interface Foo {
    myProperty?: boolean;
}

Is there a way to do this when declaring a variable? None of the following seems to work:

let myVar?: boolean;
let myVar: boolean?;
let myVar: boolean = undefined;

Upvotes: 2

Views: 2292

Answers (1)

Fenton
Fenton

Reputation: 250942

can make an entry "optional", or type | undefined, by adding a question mark

You can't use the ? syntax for this with variables.

Strict Null Checks

If you are using strict null checks (strictNullChecks compiler flag), you can explicitly allow undefined values:

let myVar: boolean | undefined;

Or nulls:

let myVar: boolean | null;

This will prevent the uninitialized variable check that the compiler performs, as shown below.

let myVar: boolean;

if (myVar) { // Hey coder! myVar hasn't been assigned!
  console.log('x');
}

If you aren't using strict null checks, the boolean type would already allow null and undefined with the normal type annotation.

let myVar: boolean;

Upvotes: 4

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