Javascript on command line

Using Node.js on your command line to quickly test JavaScript functions

Node.js is a powerful, lightweight runtime environment that allows users to run JavaScript code outside of their browser. Node has become so popular and ubiquitous that it’s become something of an essential installation on the machine of any developer working and building web applications today. While Node has hundreds of potential use cases, today we’re going to focus on a very small and specific tip that can aid your JavaScript development: using Node on your command line to quickly run a local JavaScript file and test its output. Very often when working on algorithms on LeetCode or when writing code for a project, you might find yourself wanting to quickly test a function (with your own test cases, or others) to make sure it’s working properly before submitting it as a solution or moving forward with your work. While you could write your own specific test files for your application (using something like Jest for instance), or test your functions in an online code editor, there may be circumstances where you’d like to test something quickly, easily and locally on your own machine with nothing more than a terminal window and a code editor. This is where Node comes in, short and sweet! Step one in our process is to make sure you have Node installed on your machine if you don’t already. Visit the official downloads page on the Node.js website to find steps to install the latest version of Node on your current operating system. Now, for an example let’s say for instance that you’re working on an algorithm that involves removing the duplicate instances of a number in an array as one of its steps, and you want to make sure that your function is working properly before moving on. We can do that easily right from our command line! We’ll be working in MacOS’ Terminal for this tutorial, but the process should be similar for other command line applications. First off, open Terminal and create a file. It can be anything, but we’ll use test.js: Now, let’s open that file up in our code editor! In this case we’re using Visual Studio Code, so the command to open the file will be different for other editors: Now, in our editor window we can paste or write in the function that we’d like to test. There are two key things here to note:

1. We’re creating a test case by creating a const named arr that will be passed in as our argument.
2. We’re calling the function as a console.log, a feature that normally prints something to the console in a browser; in this case our «browser» is our command line through Node!

Now we save the file and go back to our command line. Typing the following command will tell Node to open and run our test.js file:

And we’ll see the following:

We know that’s the proper expected output (in this case an array with all duplicate numbers removed), and that it worked for the test case we gave. We could change the array to whatever we wanted, or we could console.log the function multiple times with different arguments in order to test further.

The key to what we’re doing here is in that node command— you can run any JavaScript file and see its output on the command line, giving you the ability to quickly test functions locally without relying on online editors or writing separate test files within your application. Awesome!

If you’ve come this far, thanks so much for reading! I hope this little tip helps out with your own development and adds one more tool to your toolbox.

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Run Node.js scripts from the command line

The usual way to run a Node.js program is to run the globally available node command (once you install Node.js) and pass the name of the file you want to execute.

If your main Node.js application file is app.js , you can call it by typing:

Above, you are explicitly telling the shell to run your script with node . You can also embed this information into your JavaScript file with a «shebang» line. The «shebang» is the first line in the file, and tells the OS which interpreter to use for running the script. Below is the first line of JavaScript:

Above, we are explicitly giving the absolute path of interpreter. Not all operating systems have node in the bin folder, but all should have env . You can tell the OS to run env with node as parameter:

To use a shebang, your file should have executable permission. You can give app.js the executable permission by running:

While running the command, make sure you are in the same directory which contains the app.js file.

Pass string as argument to node instead of file path

To execute a string as argument you can use -e , —eval «script» . Evaluate the following argument as JavaScript. The modules which are predefined in the REPL can also be used in script.

On Windows, using cmd.exe a single quote will not work correctly because it only recognizes double » for quoting. In Powershell or Git bash, both ‘ and » are usable.

Restart the application automatically

The node command has to be re-executed in bash whenever there is a change in the application. To restart the application automatically, use the nodemon module.

Install the nodemon module globally to system path:

You can also install nodemon as a development dependency:

This local installation of nodemon can be run by calling it from within npm script such as npm start or using npx nodemon.

Run the application using the nodemon command followed by the application’s file name:

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How can I execute JavaScript at the command prompt?

This tutorial teaches us to execute JavaScript at the command prompt.

JavaScript is the most popular language among programming languages that work with the browser. It is helpful to add the behaviors to the webpage and make it attractive. So, everyone can run JavaScript efficiently in the browser, but what if they want to run JavaScript via command prompt or other terminals.

Here, we have different approaches to running JavaScript at the command prompt.

Using the Node.js

One of the most popular ways to run server-side JavaScript via command prompt is the use of Node.js. Node.js is the run time environment for JavaScript. The whole thing we need to do is download the Node.js, and install it.

Steps to Install Node.js

• Step 1 − Download the NodeJs from here according to your personal computer’s device specification.
• Step 2 − Open the installed .exe file of Node.js and install the Node.js to your local computer.
• Step 3 − To check that Node.js is correctly installed, open the terminal and type the below command. It should show you the version of the Node.js.

After successfully installing the Node.js to the local computer, you just need to create a new JavaScript file and write some code into that.

Example

The below example demonstrates to run the JavaScript code at the command prompt. The user needs to create a new file named hello.js and need to add the below code to that file.

// basic JavaScript function function runCodeFromTerminal() < let string1 = "welcome "; let string2 = "to the"; let string3 = "tutorialspoint!" console.log( string1, string2, string3 ); >// call the function on run the file. runCodeFromTerminal();

Now, to run the above code at the terminal, open the terminal and go to the folder where the current file locates. Users can use the cd command to go to the file directory.

After going to the file directory, enter the below command to the terminal.

Command

Output

Welcome to the tutorialsPoint!

Users can see that program executed successfully and prints the above output.

Write JavaScript Code to the Terminal and run it

In this method, rather than writing the JavaScript code into the file, we will write it into the terminal and directly execute it.

Make sure that you have installed Node.js on your local computer before you use this approach to run JavaScript in the command prompt.

Steps to writing code in terminal

The user must follow the steps below to write and run the JavaScript code into the terminal.

• Step 1 − Install Node.js and enter the below command to the command prompt. It will show you a welcome message in the terminal.

• Step 2 − Write the JavaScript code line by line in the terminal. Users need to remember that they can’t write JavaScript code in the command prompt as they write in the file. Users need to write code step by step, and they can press enter after only they complete the whole step. For example, if users define a function, they need to write the entire function in a single line. After that, they can press enter and call the function in a different sequence.

Example

The below example demonstrates to run the JavaScript code at the Command Prompt.

Enter every step in the terminal line by line.

// basic JavaScript function let a = 10; let b = 20; function runCodeToTerminal() < console.log( "The value of a + b is" , a+b ); >// call the function to run the file. runCodeToTerminal();

The function will call when you add the last line of the function call in your terminal and it prints the below output.

Output

Using the Nashorn Java Engine

The Nashorn is the engine based on the JSR 292 that comes with the Java SE 8. It provides a better run time performance for JavaScript.

To use the Nashorn engine, users must have installed Java on their local computer. You can find the JJS tool inside the bin folder of JDK installation, with the other tools like Jar.

We can run the JavaScript in the terminal using the JJS tool.

Syntax

Example

Users need to follow different syntax when they want to run the JavaScript using the ‘jjs’ tool. For example, they need to use print instead of console.log in JavaScript code.

Add the below code to the file called hello.js.

// basic JavaScript function to run using the JJS tool function runCodeToTerminal() < print( " Tutorial Points is the best site to learn computer science things." ); >// call the function on run the file. runCodeToTerminal();

Now, users need to run the below file using the JJS tool in the command prompt. Go to the file directory, and enter the below command to the terminal.

Command

Output

Tutorial Points is the best site to learn computer science things.

Conclusion

We have looked at the three approaches to running the JavaScript code at the command prompt. The best approach is the run JavaScript code using Node.js. If users have fewer lines of code and need to perform some testing on code, they can use the second approach. The third approach is not recommended as it doesn’t follow the standard JavaScript syntax.

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