The REPL, scripts and the browser console

There are three common places to run a line of JavaScript: an interactive prompt, a saved file, and the console inside a browser. Each suits a different job. This lesson shows all three with the same small calculation.

The REPL

Type node with no file name and Node.js starts a REPL, short for read-eval-print loop. It reads a line you type, evaluates it, prints the result and waits for the next line. It is the quickest way to check how a piece of syntax behaves.

$ node
Welcome to Node.js v26.11.1.
Type ".help" for more information.
> 2 + 3
5
> const x = 10
undefined
> x * 4
40
> _ * 2
80
> "hi".toUpperCase()
'HI'
> .exit

A few things to notice:

• Every expression prints its value. A declaration such as const x = 10 has no value, so the REPL prints undefined.
• _ holds the result of the previous expression.
• Names you declare stay available on later lines.
• .help lists the REPL’s own commands, and .exit leaves. Pressing Ctrl+D on an empty line also leaves.

Commands that start with a dot belong to the REPL. They are not JavaScript.

The REPL is for trying things. It keeps nothing, so anything worth keeping goes in a file.

Running a file

Save this as area.mjs:

const radius = 3;
const area = Math.PI * radius ** 2;

console.log(area);
console.log(area.toFixed(2));

Then run it from the folder that contains it:

node area.mjs

28.274333882308138
28.27

Math.PI is a constant that the language provides, and ** raises a number to a power. The first line shows more digits than you want: that is how JavaScript prints a number by default. area.toFixed(2) returns the number as text rounded to two decimals. The numbers lesson explains why the long form looks the way it does.

Unlike the REPL, a script prints only what you tell it to print. Without the console.log calls, the program would run and show nothing.

Re-running when the file changes

While you edit a file, you can ask Node.js to run it again each time you save:

node --watch area.mjs

--watch has been stable since Node.js 22. The process stays running and prints the output again after every save. Press Ctrl+C to stop it.

Standard output and standard error

A program has two output channels: standard output (stdout) for results, and standard error (stderr) for warnings and errors. In a terminal both appear on screen, so you rarely notice. They separate when you redirect a program’s output to a file.

console.log("result: 42");
console.error("warning: this goes to stderr");
console.log("done");

console.log writes to stdout and console.error writes to stderr. In a terminal, all three lines show:

result: 42
warning: this goes to stderr
done

If you redirect stdout into a file, with node streams.mjs > out.txt, only stderr still appears on screen, and the file receives the other two lines:

result: 42
done

The lessons’ expected outputs show stdout only, since that is what a program normally produces.

The browser console

Every browser has developer tools. Open them from the browser’s menu and choose the Console tab. It works like the REPL: you type an expression and see its value. Try 2 + 3 and "hi".toUpperCase() there as well.

Two differences from Node.js are worth knowing. The browser console gives you window and document, which Node.js does not have, and Node.js gives you process, which a browser does not have. The first lesson of this module explains why.

To run a file in a browser you need a web page that loads it. The optional browser module later in the course covers that. The Node.js route you have just used needs no page.

Common pitfalls

• Expecting a script to print the value of its last line. Only the REPL does that. In a file, use console.log.
• Running node inside the REPL. If you see > at the start of the line, you are already in the REPL. Type .exit, then run node area.mjs at the normal prompt.
• Running the file from the wrong folder. node area.mjs looks in the current folder. Cannot find module means the file is somewhere else.
• Pasting code with a leading $. The $ in examples stands for the shell prompt. Do not type it.

const width = 12;
const height = 7;

console.log(width * height);
console.log((width + height) * 2);

84
38

const boiling = 212;
const cool = 50;

console.log(((boiling - 32) * 5) / 9);
console.log(((cool - 32) * 5) / 9);

100
10

const hoursPerDay = 24;
const minutesPerHour = 60;
const secondsPerMinute = 60;

console.log(hoursPerDay * minutesPerHour * secondsPerMinute);

86400

Summary

• node alone starts the REPL for quick experiments. Leave it with .exit or Ctrl+D.
• node file.mjs runs a script, and node --watch file.mjs re-runs it on every save.
• console.log writes to stdout and console.error to stderr. Redirecting with > separates them.
• The browser’s Console tab is the same idea with different built-in names.

Next: console.log and friends.