Question

If I want to give a variable a value that it has a decent chance of already having, should I check whether it does and avoid unnecessary overwriting, or should I just do it and avoid the check? So (using JavaScript here), which option is less work for the processor:

foo = "foo"; //foo might already be set to "foo"

or

if(foo != "foo") {
    foo = "foo";
}

Two things to note here: first, while I’m working in JavaScript at the moment, I’d be interested in answers for other languages if they’re different; and second, I will most probably be working with strings that are a lot longer than “foo”.

Answer 1

Simple assignment is always going to be less costly than a condition check. That being said, this is so micro that it really does not matter.

In the case of something a little more complicated like if you only wanted to assign something to foo if it didn’t already have a value, you could use the logical or (||) to do that:

var foo;
// ... some code that may initialize foo ...
foo = foo || 'someDefault';

Beware that this will also default foo if it has any “falsey” value: 0, empty string, NaN, null, undefined, or false. If those values are acceptable then you’d want to use typeof with a tertiary statement instead:

var foo;
// ... some code that may initialize foo ...
foo = typeof foo === 'undefined' ? 'someDefault' || foo;