Typescript variable multi types

Question

I have a variable that's declared as two types. Let's take as example this:

let foo: number | number[] = null;

After that I have an if condition that assign a single number or an array to that variable:

if(condition) { 
  foo = 3;
} else {
  foo = [1,2,3];
}

The problem starts here. I can't do any action on that variable if I need to check it like if it is an array.

if(!!foo.length) { ... }

This gives me an error:

Property 'length' doesn't exists in a type number | number [].

I've red this topic: https://www.typescriptlang.org/docs/handbook/advanced-types.html#user-defined-type-guards but I was not able to make it works. I've also searched here on SO without finding anything that could have helped me.

I've kinda solved hard-casting it as any and it works, but is not an elegant solution. What am I missing?

if(!!(foo as number[]).length) { 
  // this works if foo is an array
} else {
  // this works too and I can just do something like const a:number = foo;
}

@saintlyzero sadly no, I just miss-typed here in the example. — Jacopo Sciampi
– Jacopo Sciampi, Commented Nov 26, 2019 at 10:13

Timothy · Accepted Answer · 2019-11-26 10:02:26Z

2

First think type will be as following:

let foo: null | number | number[] = null;

// OR 

foo: number | number[];

Second, you need to use type guard to narrow down types to be able to reach the variable, i.e.

if(typeof foo === 'number') { 
  foo = 3;
}
else if (typeof foo === 'object' && Array.isArray(var)) {
  foo = [1,2,3];
}
else {
// whatever
}

answered Nov 26, 2019 at 10:02

Timothy

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1 Comment

Jacopo Sciampi Over a year ago

I don't quite like it but it's working out. Thank you.

Bilel-Zheni · Accepted Answer · 2019-11-26 09:55:41Z

1

you can check if it's an array through Array.isArray(foo) :

    if (Array.isArray(foo)){ 
     // array logic
    } else {
      // number logic
}

answered Nov 26, 2019 at 9:55

Bilel-Zheni

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Comments

Aayush Gupta · Accepted Answer · 2019-11-26 09:51:29Z

0

It is a typo change lenght to length

answered Nov 26, 2019 at 9:51

Aayush Gupta

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StepUp · Accepted Answer · 2019-11-26 09:55:46Z

0

Try to check whether it is array and after you're sure that it is type of an array, then check its length:

if(Array.isArray(foo) && foo.length > 0) { 
    ... 
}

answered Nov 26, 2019 at 9:55

StepUp

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