Time of evaluation of condition (Javascript)

I'm interested at what point of time a condition in an if/else statement gets evaluated.

Imagine there's the following sample:

if (complex condition 1) {
  do something
}
else if (complex condition 2) {
  do something else
}
else if (complex condition 3) {
  do something else
}
else {
  do anything
}

What I want to know is: does each of the complex conditions get check on ahead of time, and the interpreter just execudes the codepart of the condition that is true, or does it start with the first condition when it comes to it and only evaluates the second condition when the first one is false?

I'm interested in regards to make some optimizations for low-end mobile devices.