How to pass "this" to window setInterval

Since we have ES6 now, I think we need another answer here:

Use an arrow function:

function a() {
  this.b = 1;
  this.set = setInterval(() => {console.log(this.b);}, 200);
}

Arrow functions, opposite to normal functions, don't have a this context on their own. This means you have access to the outer this.

Store a reference to this:

function a() {
    var self = this;
    self.b = 1;
    self.set = setInterval(function() {console.log(self.b);}, 200);
}

The anonymous function that you pass to setInterval has access to any variables in its containing scope, i.e., any local variables of function a(). The magic of JS closures keeps these variables alive even after a() has completed, and each invocation of a() gets its own closure.