What is a receiver in JavaScript?

The receiver is the object in which the property lookup happens.

So yes, if you use a.jump, a is the receiver.

The concept is only relevant when you can execute arbitrary code when that property lookup happens. Basically, that means:

• Accessor properties.

  You can access the receiver by using this inside the getter or setter. The receiver will usually be the object in which you defined the property, or another object which inherits from it.

  var target = {
    get getReceiver() { return this; }
  };
  target.getReceiver; // target
  var inherits = Object.create(target);
  inherits.getReceiver; // inherits
  

  A built-in example is __proto__, defined as a property of Object.prototype but expected to be got or set on other objects (receivers).

• Proxy objects

  Proxy objects allow you to define get or set traps, which run a function when you attempt to get or set any property to the proxy. The receiver is provided as an argument of that function. The receiver will usually the Proxy object itself, or an object which inherits from it.

  var proxy = new Proxy({}, {
    get: function(target, property, receiver) {
      return receiver;
    }
  });
  proxy.getReceiver; // proxy
  var inherits = Object.create(proxy);
  inherits.getReceiver; // inherits
  

Note you can use Reflect.get or Reflect.set to specify arbitrary receivers:

Reflect.get(target, "getReceiver", arbitraryValue); // arbitraryValue ¹
Reflect.get(proxy, "getReceiver", arbitraryValue); // arbitraryValue

¹ _{If the getter was defined in non-strict mode, it will be Object(arbitraryValue)}.

The name “receiver” comes from the specification, see Object Internal Methods and Internal Slots

• [[Get]]   (propertyKey, Receiver) → any

  Return the value of the property whose key is propertyKey from this object. If any ECMAScript code must be executed to retrieve the
  property value, Receiver is used as the this value when
  evaluating the code.

• [[Set]]   (propertyKey, value, Receiver) → Boolean

  Set the value of the property whose key is propertyKey to value. If any ECMAScript code must be executed to set the property value,
  Receiver is used as the this value when evaluating the code. Returns true if the property value was set or false if it
  could not be set.