April 17, 2012 0

Fetch and Iterate

By in javascript

Tonight I stumbled across a blog post from the future! Or something like that. On 4/17 at 11:30PM I read a blog post that was published on 4/18 at 1:54. Time zones = time travel. Anyway…

It was about finding a cleaner, more succinct, syntax for a standard pattern:
fetch_data, process_data, rinse and repeat until no more data!

Quick, go read the original post. Don’t worry, it’s short.

{elevator music}

Okay, so how do we improve this? Well, in some languages (like JavaScript, Ruby, PHP, et. al), an assignment expression actually returns a value. And since the expression returns a value, the expression itself can be used as the looping condition, like so:

while(data = fetch()){
  process(data);
}​

Notice the single equals sign? We are doing assignment here, not an equality check. We invoke fetch() and the result is returned and assigned to data. Now the value of data itself is used as the while condition. So the data is continually fetched and processed until fetch() returns a falsy value. (In JavaScript, that would be one of false, undefined, null, 0, "", or NaN)

This is a very common pattern in PHP, especially when fetching/querying data from a database. It’s a much less common pattern in JavaScript, but still quite valid. As is with most useful things, this approach can be misused. It can easily be confused for a typo (missing an equals sign?) so it should be used with care. For instance, ensuring that your fetch() function and data variable are named clearly should make it less likely to be misunderstood. When used appropriately, it can certainly increase the clarity of your code.

 
 

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