Command Line Arguments

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When running programs from the command line we have the option of supplying additional information in the form of arguments on the command line. Command line arguments take the form of strings separated by spaces. For example the following command line program invokation has three additional arguments.

$ nodejs Program.js Fred 123 Flintstone

In our programs we can access each of these arguments using the variables process.argv[0], process.argv[1], process.argv[2], etc. The name of the JavaScript interpreter is found in the variable with the 0 subscript. Our program filename will be in the variable with subscript 1. The user supplied arguments are in the variables with subscripts 2, 3, 4, etc.

CmdLineArgs1.js

#!/usr/bin/env node;
const Utils = require('./Utils');

const main = async () => {
	console.log("Number of arguments: " + (process.argv.length - 2));
	console.log("Program Name: " + Utils.filename(process.argv[1]));
	console.log("Arg 1: " + process.argv[2]);
	console.log("Arg 2: " + process.argv[3]);
	console.log("Arg 3: " + process.argv[4]);
	console.log("Arg 4: " + process.argv[5]);
}
main().catch( e => { console.error(e) } );

If we want to treat a command line argument as a number we first need to convert from the string representation passed on the command line to a numeric type. This is where conversion functions come in handy. The example below illustrates how to convert command line arguments to integers.

CmdLineArgs2.js

#!/usr/bin/env node;
const Utils = require('./Utils');

const main = async () => {
	const a = Utils.stoiWithDefault(process.argv[2], 0);
	const b = Utils.stoiWithDefault(process.argv[3], 0);
	const c = a + b;
	console.log(Utils.format("{0:d} + {1:d} = {2:d}", a, b, c));
}
main().catch( e => { console.error(e) } );

The example below converts the command line arguments to floating point values.

CmdLineArgs3.js

#!/usr/bin/env node;
const Utils = require('./Utils');

const main = async () => {
	const a = Utils.stodWithDefault(process.argv[2], 0);
	const b = Utils.stodWithDefault(process.argv[3], 0);
	const c = a + b;
	console.log(Utils.format("{0:f} + {1:f} = {2:f}", a, b, c));
}
main().catch( e => { console.error(e) } );

Questions

• {{Who's on first?}}
• {{Who's on second?}}
• {{Who's on third?}}

Projects

More ★'s indicate higher difficulty level.

• Combined FICA Tax (Command Line Argument)
• Cubic Roots (Command Line)
• Dog Years (Command Line)
• Milky Way Stellar Density Estimate
• Quadratic Roots (Command Line)

References

• [[JavaScript Language Reference]], Mozilla Developer Network
• [[Mozilla Developer Network]]
• Download [[node.js]]
• [[w3schools.com]]