Question

I have implemented each function like this in JS:

Object.prototype.each = function(fn){
  for (var x in this){
    if (this.hasOwnProperty(x)){
        fn(x);
    }
  }
  return this;
};


var o = {'a' : 'apple', 'b' : 'bat', 'c' : 'cat'};
o.each(function(i){
   alert(i);
});

Though this works fine. Is there anything wrong in it. Asking because I am learning JS.

Also why does it produces error if I do:

{'a' : 'apple', 'b' : 'bat', 'c' : 'cat'}.each(function(i){
   alert(i);
});

Answer 1

You need to put parentheses around the object or it will interpret it as a block due to the opening and closing curly brackets.

({'a' : 'apple', 'b' : 'bat', 'c' : 'cat'}).each(function(i){
   alert(i);
});

When you do it without the parentheses, the interpreter will “see” this:

{ // open block statement
   'a' : 'apple', 'b' : 'bat', 'c' : 'cat' // not a valid expression!
}.each(function(i){ // there's no dot property access notation for statements!
   alert(i);
});

Which is a block with a single expression. However, since that is not a valid expression it will return a syntax error. Even if it were a valid expression, it would still give you an error since blocks do not have properties and therefore the dot property access notation is not applicable:

{ console.log("hello world"); }.each(function(){})

Thus, you need to tell the interpreter that the object is an expression by wrapping it in parentheses to remove the ambiguity (since you cannot put a statement inside a parentheses – only expressions).