Code seems to work even though a semicolon is missing

Question

I have a browser javascript that extracts data from a table in a web page and creates hidden fields in a form. Below is an excerpt of the code:

var allRows = table.childNodes;
for (var i = 1 ; i < allRows.length ; i++) {
    var rowCells = allRows[i].childNodes;
    var newInput = document.createElement("input");
    var valueString = computeValue();
    newInput.setAttribute("type", "hidden");
    newInput.setAttribute("name", "timePair" + rowCells[j].getAttribute('id'))   /* This line is missing a ; */
    newInput.setAttribute("value", valueString);
}

As you can see, one function call is missing a semicolon, but the code is working anyway, with no errors in neither Internet Explorer nor Firefox.

I checked the javascript grammar and it looks like this shouldn't be allowed.

Why don't I get an error?

Answer 1

JavaScript features Automatic Semicolon Insertion :

http://jamesallardice.com/understanding-automatic-semi-colon-insertion-in-javascript/

This isn't always a good thing, for example:

return
{
    foo: function() {}
}

ASI will kick in at the return line and return void. Quite a common JS gotcha... especially for those more familiar with more "formal" languages, where a missing semicolon leads to a compilation error.

Importantly:

Note that JavaScript does not treat every line break as a semicolon: it usually treats line breaks as semicolons only if it can't parse the code without the semicolons.

Quote source: Javascript: The Definitive Guide