Question

function robot(robotId) {
  this.id = robotId;
  this.parts = new Array();

  this.collectParts = function() {
    $.getJSON('some/url', function(json) {
      for(i in json.responses) {
        this.parts = json.responses[i].parts;
      }
    });
  }
}

How do I actually assign this.parts?

Answer 1

Assign a reference to this (when it’s in the proper scope) to a variable and use that variable in the function which has changed the scope of this. In the modified version of your code below, robotInstance is the variable I’ve opted to use:

function robot(robotId) {
  var robotInstance = this;

  this.id = robotId;
  this.parts = new Array();

  this.collectParts = function() {
    $.getJSON('some/url', function(json) {
      for(i in json.responses) {
        robotInstance.parts = json.responses[i].parts;
      }
    });
  }
}

Edit: I had written this modification last night, then decided not to post it. But based on the comment to your question by @Ӫ_._Ӫ, I decided I’d show you the way I would write your code:

var robot = function( robotId )
{
  this.id = robotId;
  this.parts = [];
};
robot.prototype = {
  collectParts: function()
  {
    var robotInstance = this;

    $.getJSON(
      'some/url',
      function( json )
      {
        var i,
            responses = json.responses;
        for( i in responses )
        {
          if( Object.prototype.hasOwnProperty.call( responses, i ) )
          {
            robotInstance.parts = responses[i].parts;
          }
        }
      }
    );
  }
};