Things You Should Never Do, Part I – Joel on Software

I’m Joel Spolsky, a software developer in New York City. More about me.
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Netscape 6.0 is finally going into its first public beta. There never was a version 5.0. The last major release, version 4.0, was released almost three years ago. Three years is an awfully long time in the Internet world. During this time, Netscape sat by, helplessly, as their market share plummeted.
It’s a bit smarmy of me to criticize them for waiting so long between releases. They didn’t do it on purpose, now, did they?
Well, yes. They did. They did it by making the single worst strategic mistake that any software company can make:
They decided to rewrite the code from scratch.
Netscape wasn’t the first company to make this mistake. Borland made the same mistake when they bought Arago and tried to make it into dBase for Windows, a doomed project that took so long that Microsoft Access ate their lunch, then they made it again in rewriting Quattro Pro from scratch and astonishing people with how few features it had. Microsoft almost made the same mistake, trying to rewrite Word for Windows from scratch in a doomed project called Pyramid which was shut down, thrown away, and swept under the rug. Lucky for Microsoft, they had never stopped working on the old code base, so they had something to ship, making it merely a financial disaster, not a strategic one.
We’re programmers. Programmers are, in their hearts, architects, and the first thing they want to do when they get to a site is to bulldoze the place flat and build something grand. We’re not excited by incremental renovation: tinkering, improving, planting flower beds.
There’s a subtle reason that programmers always want to throw away the code and start over. The reason is that they think the old code is a mess. And here is the interesting observation: they are probably wrong. The reason that they think the old code is a mess is because of a cardinal, fundamental law of programming:
It’s harder to read code than to write it.
This is why code reuse is so hard. This is why everybody on your team has a different function they like to use for splitting strings into arrays of strings. They write their own function because it’s easier and more fun than figuring out how the old function works.
As a corollary of this axiom, you can ask almost any programmer today about the code they are working on. “It’s a big hairy mess,” they will tell you. “I’d like nothing better than to throw it out and start over.”
“Well,” they say, “look at this function. It is two pages long! None of this stuff belongs in there! I don’t know what half of these API calls are for.”
Before Borland’s new spreadsheet for Windows shipped, Philippe Kahn, the colorful founder of Borland, was quoted a lot in the press bragging about how Quattro Pro would be much better than Microsoft Excel, because it was written from scratch. All new source code! As if source code rusted.
The idea that new code is better than old is patently absurd. Old code has been used. It has been tested. Lots of bugs have been found, and they’ve been fixed. There’s nothing wrong with it. It doesn’t acquire bugs just by sitting around on your hard drive. Au contraire, baby! Is software supposed to be like an old Dodge Dart, that rusts just sitting in the garage? Is software like a teddy bear that’s kind of gross if it’s not made out of all new material?
Back to that two page function. Yes, I know, it’s just a simple function to display a window, but it has grown little hairs and stuff on it and nobody knows why. Well, I’ll tell you why: those are bug fixes. One of them fixes that bug that Nancy had when she tried to install the thing on a computer that didn’t have Internet Explorer. Another one fixes that bug that occurs in low memory conditions. Another one fixes that bug that occurred when the file is on a floppy disk and the user yanks out the disk in the middle. That LoadLibrary call is ugly but it makes the code work on old versions of Windows 95.
Each of these bugs took weeks of real-world usage before they were found. The programmer might have spent a couple of days reproducing the bug in the lab and fixing it. If it’s like a lot of bugs, the fix might be one line of code, or it might even be a couple of characters, but a lot of work and time went into those two characters.
When you throw away code and start from scratch, you are throwing away all that knowledge. All those