Q:

name : function(){} or function name (){} | ReactJS

I am new to ReactJS and as I am learning I noticed that there's a few ways to declare functions such as,
MyFunc : function(){
  ///
}

or
function MyFunc(){
  ///
}

I was wondering which one shall i use and why?

A:

This is not a React specific way of declaring functions. It is a javascript way.
MyFunc : function(){
  ///
}

Would be in an object literal and
function MyFunc(){
 ///
}

Would be a constructor or general function.
For example
const someObject = {
  MyFunc: function() {
    // some definition here.
  }
}

and
function MyFunc {
  // some definition here. Something like this.name = 'blah'
}

Which one shall I use and why

That would depend on what behavior you would like. With the constructor function notation, you create an object that can be instantiated into multiple instances (with the new keyword), while the literal notation delivers a single object, like a singleton.

