Compiled vs. Interpreted

You may have heard of code compilation before, but perhaps it seems like a mysterious black box where source code slides in one end and executable programs pop out the other.

It’s not mysterious or magical, though. Code compilation is a set of steps that process the text of your code and turn it into a list of instructions the computer can understand. Typically, the whole source code is transformed at once, and those resulting instructions are saved as output (usually in a file) that can later be executed.

You also may have heard that code can be interpreted, so how is that different from being compiled?

Interpretation performs a similar task to compilation, in that it transforms your program into machine-understandable instructions. But the processing model is different. Unlike a program being compiled all at once, with interpretation the source code is transformed line by line; each line or statement is executed before immediately proceeding to processing the next line of the source code.

Fig. 1: Compiled vs. Interpreted Code

Figure 1 illustrates compilation vs. interpretation of programs.

Are these two processing models mutually exclusive? Generally, yes. However, the issue is more nuanced, because interpretation can actually take other forms than just operating line by line on source code text. Modern JS engines actually employ numerous variations of both compilation and interpretation in the handling of JS programs.

Recall that we surveyed this topic in Chapter 1 of the Get Started book. Our conclusion there is that JS is most accurately portrayed as a compiled language. For the benefit of readers here, the following sections will revisit and expand on that assertion.