Typescript: default value of function parameter missing required properties

Question

interface IPrintConfig {
    name: string;
    copies: number;
}

function print(config: IPrintConfig = {}) {
    console.log(`Print ${config.copies} on printer ${config.name}`);
}

print();

when nothing it passed to print() i made the default value as {}. But print() is expecting a value of type IPrintConfig which comntains name and copies properties. I can solve this issue by making both of those properties as optional. Is there another way to solve this issue without making them optional

What values do you want config.copies and config.name to have inside the implementation if someone just calls print()? — jcalz
– jcalz, Commented Jul 10, 2019 at 18:19

Get Off My Lawn · Accepted Answer · 2019-07-10 18:37:06Z

4

Option 1

Make your interface values optional using a ?:

interface IPrintConfig {
    name?: string;
    copies?: number;
}

function print(config: IPrintConfig = {}) {
    console.log(`Print ${config.copies} on printer ${config.name}`);
}

Option 2

Make the parameter optional instead of predefined:

function print(config?: IPrintConfig) {
  if(config)
    console.log(`Print ${config.copies} on printer ${config.name}`);
  else
    console.log(`No value passed`);
}

Option 3

Set the default values:

function print(config: IPrintConfig = {name: '', copies: -1}) {
    console.log(`Print ${config.copies} on printer ${config.name}`);
}

Option 4

Require that the parameter be passed to the function:

function print(config: IPrintConfig) {
    console.log(`Print ${config.copies} on printer ${config.name}`);
}

edited Jul 10, 2019 at 18:37

answered Jul 10, 2019 at 18:17

Get Off My Lawn

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1 Comment

Get Off My Lawn Over a year ago

Yeah, you wouldn't assign a default value.

Medet Tleukabiluly · Accepted Answer · 2019-07-10 18:35:23Z

0

What if you make param optional?

interface IPrintConfig {
    name: string;
    copies: number;
}

function print(config?: IPrintConfig) {
    config  && console.log(`Print ${config.copies} on printer ${config.name}`);
}

print();

edited Jul 10, 2019 at 18:35

answered Jul 10, 2019 at 18:21

Medet Tleukabiluly

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8 Comments

jcalz Over a year ago

What do you intend config?.properties to mean? That's not valid TypeScript or JavaScript syntax, as far as I know

Medet Tleukabiluly Over a year ago

Indeed it is a valid operator in TypeScript, and called optional chaining

jcalz Over a year ago

I don't think so

Get Off My Lawn Over a year ago

The exclamation point is valid though config!.properties might mean something different though

jcalz Over a year ago

config!.properties will throw a runtime exception if config is not defined, so you don't want to do that.

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