Typescript: Difference between => and : in interface function declaration

Question

What is the difference between defining a function in an interface like:

interface IMyInterface1 {
    functionName(value: string): void;
}

and

interface IMyInterface2 {
    functionName: (value: string) => void;
}

Answer 1

There's no real difference here, you can use which one you feel more comfortable with.
With that being said, I consider the first to imply a class method while the second is a member which is a function:

class MyClass1 implements IMyInterface1 {
    functionName(value: string): void {}
}

class MyClass2 implements IMyInterface2 {
    functionName = function(value: string): void {}
}

It is just a personal preference, the compiler will let you do the opposite as well:

class MyClass1 implements IMyInterface1 {
    functionName = function(value: string): void {}
}

class MyClass2 implements IMyInterface2 {
    functionName(value: string): void {}
}

In a similar way, these two notations are the same:

let obj1 = {
    functionName() {}
}

let obj2 = {
    functionName: function() {}
}

As they are both compiled to:

var obj1 = {
    functionName: function () { }
};
var obj2 = {
    functionName: function () { }
};