How to define optional constructor arguments with defaults in Typescript

Question

Is it possible to have optional constructor arguments with default value, like this

export class Test {
    constructor(private foo?: string="foo", private bar?: string="bar") {}
}

This gives me the following error:

Parameter cannot have question mark and initializer.

I would like to create instances like

x = new Test();               // x.foo === 'foo'            
x = new Test('foo1');         // x.foo === 'foo1'
x = new Test('foo1', 'bar1');

What is the correct typescript way to achieve this?

Nitzan Tomer · Accepted Answer · 2017-04-10 14:54:24Z

115

An argument which has a default value is optional by definition, as stated in the docs:

Default-initialized parameters that come after all required parameters are treated as optional, and just like optional parameters, can be omitted when calling their respective function

It's the same for constructors as it is for other functions, so in your case:

export class Test {
    constructor(private foo: string = "foo", private bar: string = "bar") {}
}

answered Apr 10, 2017 at 14:54

Nitzan Tomer

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3 Comments

Andre Elrico Over a year ago

private foo = "foo", private bar = "bar" I guess the type can be inferred here.

Derek Lawrence Over a year ago

in my own personal preference I liketo always type define even when it's obvious

Dan Dascalescu Over a year ago

How do you do this with destructured parameters, as succintly as possible, without duplicating the list of destructured fields?

csandreas1 · Accepted Answer · 2020-11-07 13:32:28Z

8

You can add question mark after your args, which is cleaner. If you add default parameters, it should be optional by default.

export class Test {
   foo: string; 
   bar: string;

    constructor(foo?: string, bar?: string) {
    this.foo = foo;
    this.bar = bar;
   }
}

answered Nov 7, 2020 at 13:32

csandreas1

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4 Comments

Burhan Ali Over a year ago

Unclear if this worked at the time, but now compilation fails with this error: error TS2322: Type 'string | undefined' is not assignable to type 'string'.

crobbo Over a year ago

Don't agree with comment that variables with default parameter "should be optional by default". Default parameters are to provide a default value to a variable being passed into a function. A variable that is optional does NOT need to have a default value.

Ashish Kamble Over a year ago

@BurhanAli inside constructor this.foo = foo!; this.bar = bar!; will fix this

Mack Aug 11 at 16:00

doing that is not type safe, the caller can provide a value of undefined and you assert it away. An optional parameter with ? is not the same as providing a default

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