Dloggist

Function binding When passing object methods as callbacks, for instance to  setTimeout , there’s a known problem: "losing  this ". In this chapter we’ll see the ways to fix it. Losing “this” We’ve already seen examples of losing  this . Once a method is passed somewhere separately from the object –  this  is lost. Here’s how it may happen with  setTimeout : let user = { firstName : "John" , sayHi ( ) { alert ( ` Hello, ${ this . firstName } ! ` ) ; } } ; setTimeout ( user . sayHi , 1000 ) ; // Hello, undefined! As we can see, the output shows not “John” as  this.firstName , but  undefined ! That’s because  setTimeout  got the function  user.sayHi , separately from the object. The last line can be rewritten as: let f = user . sayHi ; setTimeout ( f , 1000 ) ; // lost user context The method  setTimeout  in-browser is a little special: it sets  this=window  for the function call (for Nod...