Meaning of forEach(function(tag) in JavaScript

Question

I came across the following code fragment in a CouchDB book.

function(doc) {
  doc.tags && doc.tags.forEach(function(tag) {
    emit(tag, 1);
  });
}

Can some one explain how does the function(tag) part works?

Thanks and regards,

raj

Answer 1

This is called an anonymous inline function expression. It creates a function and gives you a reference to it, similar to if you had written:

function emitTag(tag) {
    emit(tag, 1);
}
doc.tags && doc.tags.forEach(emitTag);

The array.forEach method calls the given function once for each of the items in array in order. It is a standard method in ECMAScript Fifth Edition and has been in many browsers for some time, but not JScript (IE). I am guessing couchdb takes care of that issue for you though.

Answer 2

function(tag) {...} 

gets called "for each" tag in "doc.tags" with the "tag" argument passed to the lambda function in question.

Answer 3

forEach simply iterates over array and calls function you pass to it with every element it finds.

Be aware that not every browser support it, there's helper function $.forEach in jQuery, it is safer in terms of browser support.