Computers have many senses -- keyboard, mouse, network card, camera, joystick, etc. Collectively, these are called INPUT.
Computers can also express themselves in many ways -- text, graphics, sound, networking, printing, etc. Collectively, these are called OUTPUT.
Input and Output together are called I/O.
In JavaScript,
console.log means "print a line to the terminal"In NodeJS,
process.stdin means "input coming from the terminal"process.stdin.on('data', (chunk) => { console.log(chunk) })
process.stdin.on('data', (chunk) => { console.log(chunk) })
| phrase | meaning |
|---|---|
process.stdin |
hey terminal input, |
.on('data', ... )
|
when you get some data, |
(chunk) |
please name it chunk
|
=> |
and send it to |
{ ... }
|
this block of code |
console.log(chunk) |
print chunk to the terminal |
=> is called "fat arrow" and is equivalent to this:
process.stdin.on('data', function(chunk) { console.log(chunk) })
and the block of code itself is called a callback (since you are asking stdin to call you back when it receives input).
hello.js in your text editorChange it to contain the following code:
console.log("What is your name?"); process.stdin.on('data', (chunk) => { let name = chunk.toString(); console.log("Hello, " + name + "!"); });
Save the file and switch back to the terminal
Run the program using node hello.js
Type in your name and press the RETURN (or ENTER) key
What happens? Is this what you expected?
Uh-oh! We've got trouble... what is that exclamation point doing way down there?
The first thing to do is DON'T PANIC!
You are totally going to figure this out.
And even if you don't, you haven't actually broken anything.
In fact, it's really hard to break a computer just by typing, so stay calm.
trim to a string, it will remove all SPACES and NEWLINES from both endsChange the program to look like this:
console.log("What is your name?"); process.stdin.on('data', (chunk) => { let name = chunk.toString().trim(); console.log("Hello, " + name + "!"); });
Run it and make sure it works OK
Hint: remember slice ?
This is a very big topic, but briefly...
Traditional programs are written using sequences, which are performed in order like a traditional recipe, performed by a single chef:
Even though the recipe is written in a strict order, many of these steps can happen simultaneously or in parallel.
For instance, you don't have to wait for the oven to be preheated before rolling out the dough.
Or, you could have one cook rolling, cutting, and baking, and another cook removing and decorating.
NodeJS programs are written using events, which is like a bunch of cooks, each performing one part of the recipe.
The source code of the evented cookie baking program in the previous slide could just as well be written like this:
To force events to happen in order you may need to nest your callbacks.
console.log("what is your name?")
process.stdin.once('data', (name) => {
console.log("what is your quest?")
process.stdin.once('data', (quest) => {
console.log("what is your favorite color?")
process.stdin.once('data', (color) => {
console.log("Hello " + name + "! " +
"Good luck with " + quest +
"and here is a " + color + " flower for you.");
process.exit();
});
});
});
Evented programs are often more flexible and high-performance than traditional sequenced programs, but they can be more confusing for humans to write and to read (and to debug!).
Also, sequences naturally end when they are finished, but evented programs will just keep doing the same things over and over again, as long as the triggers keep happening.
This means that you may need to explicitly call process.exit() in NodeJS programs.
If you have a simple sequence in mind, and want to emulate it using an evented system, you could use the following technique:
Set up your event responders to happen only once.
In NodeJS, this is accomplished by sending the once message in place of the on message.
console.log("What is your name?"); process.stdin.once('data', (chunk) => { let name = chunk.toString().trim(); console.log("Hello, " + name + "!"); process.exit(); // don't forget to stop! });
name.js that asks two things:
node name.js on the command line.You just wrote a program!
You are now officially a coder. HIGH FIVE!
name.js so it also prints the number of characters in the user's name.For instance:
What is your first name? Grace What is your last name? Hopper Hello, Grace Hopper! Your name is 11 characters long.
/
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