TypeScript - No "Variable is used before being assigned" inside a function

user_1332509 0
Viewed 924

I would like to understand the following TypeScript behavior:

The following code

let a: number
if (a === undefined) {
    console.log("how?")
}

throws an error: "Variable 'a' is used before being assigned.".

But the following code

let a: number
const f = (): void => {
    if (a === undefined) {
        console.log("how?")
    }
}
f()

works just fine and logs "how?".

Why is that? And also, how come a === undefined if its type is number?

typescript let initialization undefined variables
3 Answers

There's an open issue about this in the Typescript GitHub project. It's a limitation of typescript's technical design.

From Ryan Cavanaugh, it's

because we don't inline the flow-control effects of functions

user_21781590

To deal with the error Variable 'a' is used before being assigned. the simple solution I think is to declare the variable as either number or undefined, instead of just number.

let a: number | undefined

One other pattern is just to initialize a variable with a value:

let a: number = 0

I used that pattern a lot in C# years ago, but I don't personally care for that pattern in JavaScript and prefer what you have of leaving a variable un-set if the conditions to assign it a value are conditional.

user_20222080

For the second question: in TS, variables not initialized are always undefined, despite of their type. This is different, for example, from C# where a variable typed bool is always true or false and initialized as false. In TS that variable could be also undefined and, if I remember well, also null. So sometimes is good to write 

if( testVariable === false)

instead of simply

if( !testVariable )
user_53841650
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