Question

Are there any performance issues with passing an argument as an expression, instead of first making it a variable?

someFunction( x+2 );

vs.

var total = x+2;
someFunction( total );

And how about functions?

someFunction( someOtherFunction() );

Answer 1

Though the difference is minimal, the answer is really implementation-specific; JavaScript engines almost certainly differ in how they allocate things. However, I can tell you that most likely, the differences are similar to what they would be in most other languages of which I can examine the memory and processor registers in the debugger. Let’s examine one scenario:

var sum = x+2;
someFunction(sum);

This allocates memory to hold sum, which hangs around as long as the function is in scope. If the function ends up being a closure, this could be forever. In a recursive function this could be significant.

someFunction(x+2);

In most languages, this will compute x+2 on the stack and pass the result to someFunction. No memory is left hanging around.

The answer would be exactly the same for a function return value.

So in summary:

1. The exact answer depends on the JavaScript engine’s implementation.

2. Most likely you won’t notice a performance difference.

3. You may want to use variables when the result is re-used, or, when you want to examine the result easily in the debugger.

It’s mostly a matter of personal preference.