What Is REPL?

A Read-Eval-Print Loop, or REPL, is a computer environment where user inputs are read and evaluated, and then the results are returned to the user. REPLs provide an interactive environment to explore tools available in specific environments or programming languages. Some examples include the Node.js console, IPython, the Bash shell, and the developer console found in most web browsers.

To illustrate how a Read-Eval-Print Loop works, consider the following example of a Bash shell on an Ubuntu server:

To interact with the server, a user enters commands that instruct the server to perform specific actions or return certain information. For instance, a user may execute the expr command, which is used to evaluate mathematical expressions and perform calculations. In this example, the expr command evaluates the expression 2 + 2:

expr 2 + 2

The Bash shell reads the expr command as entered by the user, evaluates it, and then prints a response:

Output
4

Following that, the Bash shell prompt returns to read mode, thereby closing the loop and allowing the user to run another command:

If you’d like to learn more about REPLs and get experience using one directly, we encourage you to check out this tutorial on How To Use the Node.js REPL