Typescript - Default parameters on class with interface

Question

I have a scenario where I have an interface which has a method like so:

interface SomeInterface
{
   SomeMethod(arg1: string, arg2: string, arg3: boolean);
}

And a class like so:

class SomeImplementation implements SomeInterface
{
   public SomeMethod(arg1: string, arg2: string, arg3: boolean = true){...}
}

Now the problem is I cannot seem to tell the interface that the 3rd option should be optional or have a default value, as if I try to tell the interface there is a default value I get the error:

TS2174: Default arguments are not allowed in an overload parameter.

If I omit the default from the interface and invokes it like so:

var myObject = new SomeImplementation();
myObject.SomeMethod("foo", "bar");

It complains that the parameters do not match any override. So is there a way to be able to have default values for parameters and inherit from an interface, I dont mind if the interface has to have the value as default too as it is always going to be an optional argument.

Ryan Cavanaugh · Accepted Answer · 2013-09-05 15:45:02Z

79

You can define the parameter to be optional with ?:

interface SomeInterface {
     SomeMethod(arg1: string, arg2: string, arg3?: boolean);
}

answered Sep 5, 2013 at 15:45

Ryan Cavanaugh

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4 Comments

Grofit Over a year ago

oh I thought this was removed in favor of the default value in one of the recent versions. So I am able to use the optional argument in the interface and default value argument in the class?

Ryan Cavanaugh Over a year ago

You can still use optional arguments without defaults anywhere. The only disallowed thing is the redundant syntax x ?= 4 -- you can either have the ? or the default value.

vegemite4me Over a year ago

And beware that if you call SomeMethod('foo','bar') the arg3 parameter will be undefined, and not null.

pensan Over a year ago

And also, if you have a optional argument, all following arguments must also be optional. So arg4: boolean is not allowed.

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