Typescript – Defining TypeScript callback type

callbacktypestypescript

I've got the following class in TypeScript:

class CallbackTest
{
    public myCallback;

    public doWork(): void
    {
        //doing some work...
        this.myCallback(); //calling callback
    }
}

I am using the class like this:

var test = new CallbackTest();
test.myCallback = () => alert("done");
test.doWork();

The code works, so it displays a messagebox as expected.

My question is: Is there any type I can provide for my class field myCallback? Right now, the public field myCallback is of type any as shown above. How can I define the method signature of the callback? Or can I just set the type to some kind of callback-type? Or can I do nether of these? Do I have to use any (implicit/explicit)?

I tried something like this, but it did not work (compile-time error):

public myCallback: ();
// or:
public myCallback: function;

I couldn't find any explanation to this online, so I hope you can help me.

Best Answer

I just found something in the TypeScript language specification, it's fairly easy. I was pretty close.

the syntax is the following:

public myCallback: (name: type) => returntype;

In my example, it would be

class CallbackTest
{
    public myCallback: () => void;

    public doWork(): void
    {
        //doing some work...
        this.myCallback(); //calling callback
    }
}

As a type alias:

type MyCallback = (name: type) => returntype;

Related Solutions

Python – What’s the canonical way to check for type in Python

To check if o is an instance of str or any subclass of str, use isinstance (this would be the "canonical" way):

if isinstance(o, str):

To check if the type of o is exactly str (exclude subclasses):

if type(o) is str:

The following also works, and can be useful in some cases:

if issubclass(type(o), str):

See Built-in Functions in the Python Library Reference for relevant information.

One more note: in this case, if you're using Python 2, you may actually want to use:

if isinstance(o, basestring):

because this will also catch Unicode strings (unicode is not a subclass of str; both str and unicode are subclasses of basestring). Note that basestring no longer exists in Python 3, where there's a strict separation of strings (str) and binary data (bytes).

Alternatively, isinstance accepts a tuple of classes. This will return True if o is an instance of any subclass of any of (str, unicode):

if isinstance(o, (str, unicode)):

Python – How to determine a Python variable’s type

Use the type() builtin function:

>>> i = 123
>>> type(i)
<type 'int'>
>>> type(i) is int
True
>>> i = 123.456
>>> type(i)
<type 'float'>
>>> type(i) is float
True

To check if a variable is of a given type, use isinstance:

>>> i = 123
>>> isinstance(i, int)
True
>>> isinstance(i, (float, str, set, dict))
False

Note that Python doesn't have the same types as C/C++, which appears to be your question.

Best Answer

Related Solutions

Python – What’s the canonical way to check for type in Python

Python – How to determine a Python variable’s type

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