15: If and Else

You already learned about jumps in Exercise 11 where you wrote a function that caused your code to jump to the top of the function, and then return to where you called the function. If you don't remember this then please go back to Exercise 11 and study it again because we are going to use the concept of jumping in your code to explain an if-statement. Before you can learn about if-statements you must first learn about "testing", which involves comparing two values or variables to get a true or false answer.

If-Statements

The easiest way to talk about if-statements is to show you a small snippet of code and then show you how that relates to testing and jumps:

if (x === 10) {
   // first jump
} else if (x < 10) {
   // second jump
} else {
   // last jump
}

Here's how this code works:

1. An if-statement like this simply performs the tests in (..), and if the test is true, then it runs the code in the {..} after.
2. If the test is false, then it jumps to the next else if and performs that test.
3. If that test is true it runs the {..}, otherwise it jumps to the next else if.
4. When it has run out of else if clauses, the if-statement finally runs the code in the {..} after the else clause. Remember that the else only runs if all of the previous parts are false.

The Code

Now that you have an idea of how an if-statement works you can type in this code and play with it:

let x = 20;

if (x === 10) {
  console.log("x is equal to 10:", x);
} else if (x < 10) {
  console.log("x is less than 10:", x);
} else {
  console.log("x is greater than 10:", x);
}

// consider this part a puzzle to solve
// what's the least number of times you have to change
// the code to make each console.log run?
let y = 100;

if (y == 100) {
    if(x < 10) {
        console.log("log 1", x, y);
    } else {
        console.log("log 2", x, y);
    }
} else {
    if(x > 10) {
        console.log("log 3", x, y);
    } else {
        console.log("log 4", x, y);
    }
}

You'll see that there's a little puzzle in the second half where I've created a complicated if-statement with an if-statement inside. You should try to solve the puzzle, but be warned that I mean for you to edit this file multiple times to get each console.log to run. It's not possible to set x and y in a way that would cause all branches of those if-statements to run.

What You Should See

You should change this code to change the variables and see how that changes the output. With the code as it is above the output should be:

$ node "code.js" 
x is greater than 10: 20
log 2 20 100