How to assign string | undefined to string in TypeScript?

Question

I want to assign a variable, which is string | undefined, to a string variable, as you see here:

private selectedSerialForReplace(): string | undefined {
    return this.selectedSerials.pop();
  }

luminaireReplaceLuminaire(params: {  "serial": string; "newserial": string; }, options?: any): FetchArgs {
............
}

luminaireReplaceLuminaire({serial: this.selectedSerialForReplace(), newserial: response.output});

I get this error:

Argument of type '{ serial: string | undefined; newserial: any; }' is not assignable to parameter of type '{ "serial": string; "newserial": string; }'

I cannot change selectedSerialForReplace() function to return anything else. Could you please help me?

Please try to improve your minimal reproducible example with reproducible code. What is the logic behind luminaireReplaceLuminaire? Is it supposed to accept undefined "serial" parameters? — E_net4
– E_net4, Commented Jul 19, 2017 at 15:36
You can specify type for useState(), e.g: const [username, setUsername] = useState<string | null>(null); — Eric
– Eric, Commented Apr 6, 2024 at 23:07

Acevail · Accepted Answer · 2017-07-19 15:35:24Z

46

The typescript compiler performs strict null checks, which means you can't pass a string | undefined variable into a method that expects a string.

To fix this you have to perform an explicit check for undefined before calling luminaireReplaceLuminaire().

In your example:

private selectedSerialForReplace(): string | undefined {
    return this.selectedSerials.pop();
}

luminaireReplaceLuminaire(params: {  "serial": string; "newserial": string; }, options?: any): FetchArgs {
    ............
}

const serial = this.selectedSerialForReplace();
if(serial !== undefined) {
    luminaireReplaceLuminaire({serial, newserial: response.output});
}

answered Jul 19, 2017 at 15:35

Acevail

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1 Comment

Dagrooms Over a year ago

Side note, it's important to use != when checking undefined to catch null as well. null == undefined is true, null === undefined is false

sschoof · Accepted Answer · 2020-02-21 11:58:56Z

40

If you are sure that serial could not be undefined you can use the ! post-fix operator

luminaireReplaceLuminaire({serial: this.selectedSerialForReplace()!, newserial: response.output});

edited Feb 21, 2020 at 11:58

answered Nov 29, 2017 at 13:27

sschoof

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1 Comment

WestFarmer Over a year ago

any quick solution if it's array of [string | undefined ] to [ string ]

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