Question

I am looking for var self = this alternative plan.

var Animal = function(name){
  this.name = name;
  this.arr = [1,2,3,4];
  this.inc = function(num){
      return num + 1;
  };

  this.fireArr = function(){
    var self = this;
    this.arr.forEach(function(item){
      console.log(self.inc(item));
    });

  };

};

var dog = new Animal("dog");
console.log(dog.fireArr());

My fiddle is here.

http://jsfiddle.net/haradashinya/TtYpc/

Do you have any idea?

Thanks in advance.

Answer 1

You can use .bind() to make sure the function is called with the right this value:

function fireArr() {
    this.arr.forEach(function(item){
        console.log(this.inc(item));
    }.bind(this));
}

But imho the self (that, _this) variable is easier to understand, because it directly states that not the normal this value is used, although one would expect it (e.g. in an event handler, or jQuery’s each()). Especially on long functions, where you don’t see the bind() in the end, this is of importance. Also, some ancient browsers do not support bind() and you would need to shim it.

So, for any in-place function expressions I recommend the use of a dereferencing variable.

But it can be of great use when you have a method defined somewhere, normally using this to point to the current object as it is common in that context, and then the method should be used somewhere else. Instead of a var self-wrapper, you can and should use bind for simplicity and clarity. Your example offers quite a good demo (assuming the inc method used the this keyword):

this.arr.forEach( this.inc.bind(this) );

(although forEach() allows us to pass a custom this argument – event attachers for example don’t)