couchdb - Meaning of forEach(function(tag) in JavaScript - Stack Overflow

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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

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

• javascript
• couchdb

Share Improve this question Follow edited Oct 23, 2009 at 2:22 Sasha Chedygov 131k2626 gold badges106106 silver badges116116 bronze badges asked Oct 22, 2009 at 11:08 Rajkumar SRajkumar S 2,52166 gold badges2323 silver badges2929 bronze badges

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3 Answers 3

Sorted by: Reset to default 9

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.

function(tag) {...} 

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

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.

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