javascript

Chapter 20: Best Practices
(continued)
do {
process(values[i++]);
process(values[i++]);
process(values[i++]);
process(values[i++]);
process(values[i++]);
process(values[i++]);
process(values[i++]);
process(values[i++]);
} while (--iterations > 0);
In this implementation, the leftover count that wouldn ’ t have been handled in the loop when simply
dividing by 8 is handled in an initial loop. Once those extra items are processed, execution continues in
the main loop that calls process() eight times. This approach is almost 40% faster than the original
Duff ’ s device implementation.
Unrolling loops can yield big savings for large datasets but may not be worth the extra effort for small
datasets. The trade - off is that it takes more code to accomplish the same task, which is typically not
worth it when large datasets aren ’ t being processed.
Avoid Double Interpretation
Double interpretation penalties exist when JavaScript code tries to interpret JavaScript code. This
situation arises when using the eval() function or the Function constructor, or when using
setTimeout() with a string argument. Here are some examples:
//evaluate some code - AVOID!!
eval(“alert(‘Hello world!’)”);
//create a new function - AVOID!!
var sayHi = new Function(“alert(‘Hello world!’)”);
//set a timeout - AVOID!!
setTimeout(“alert(‘Hello world!’)”, 500);
In each of these instances, a string containing JavaScript code has to be interpreted. This can ’ t be done
during the initial parsing phase because the code is contained in a string, which means a new parser has
to be started while the JavaScript code is running to parse the new code. Instantiating a new parser
has considerable overhead, so the code runs slower than if it were included natively.
There are workarounds for all of these instances. It ’ s rare that eval() is absolutely necessary, so try to
avoid it whenever possible. In this case, the code could just be included inline. For the Function
constructor, the code can be rewritten as a regular function quite easily, and the setTimeout() call can
pass in a function as the first argument. Here are some examples:
//fixed
alert(‘Hello world!’);
//create a new function - fixed
var sayHi = function(){
alert(‘Hello world!’);
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