Run a JavaScript file

Put code in a file and execute it

⏱ Est. ~6 min

01 · Read

The REPL is good for experiments, but real programs live in files. A JavaScript file is just a text file with the .js extension. You write code into a file, then tell Node to run it.

Every real app works this way — from tiny scripts to massive web apps. Code lives in files. You run the files.

02 · Real-machine exercise

Use the terminal to create a JavaScript file called hello.js. This command writes a single line of JavaScript into a new file.

echo 'console.log("Hello from a file!")' > hello.js

03 · Real-machine exercise

Now run your file with Node. This tells Node to read hello.js and execute every line of JavaScript in it.

node hello.js

04 · Quiz

What command runs a file called app.js with Node?

javascript app.js
run app.js
npm app.js
node app.js

05 · Fill in the blank

The development loop is: write code, _____ it, check the output.

06 · Read

You just completed the write-run-test loop — the foundation of all software development:

1. Write code into a file 2. Run the file 3. See the result 4. Repeat

Every engineer, from a beginner to a senior at Google, does this loop thousands of times a day. The code gets more complex, but the loop stays the same.

Key points

JavaScript files have the .js extension
Run them with node filename.js
The write-run-test loop is the heartbeat of development

07 · Quiz

You create a file called greet.js, run node greet.js, but nothing shows up in the terminal. What's the most likely reason?

The file exists but doesn't have any console.log() or output statements
Node.js can't read files with that name
You should run npm greet.js instead of node greet.js
JavaScript files need a .node extension to execute

Other lessons in this chapter

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