To create a for loop, we use the keyword for (which means you can't use the word "for" for variable names as it already has a job in Apps Script). We then need to give some information with our loop so the computer knows how many times to loop through the code in the loop. To do this, we create a counting variable and give it an initial starting value, and then define how the counting happens and when the counting (and the loop) should stop. Let's take a look at an example:

for (var i = 0; i < 5; i++) {
    // code that runs inside the loop
}

In the above example, we have three pieces of information inside the brackets, separated by semi-colons.

1. The first part var i = 0 defines a new variable, i, and gives it an initial value of 0. Your counting variable doesn't have to start from 0 - it might be more useful to start it from another number. For example, if you are working through values in a 2D array and the first sub-array is the header row of the spreadsheet, you might want to start from the 2nd array, which would be at index number 1. Therefore, you could set your value of i to be 1. It also doesn't have to be called i, though this is typically used - you can give it any variable name you'd like.
2. The second part i < 5 is the condition that is tested each time the loop runs to check whether the loop should run again. In this example it checks whether the counting variable i is less than 5. If this condition is true, the loop will run again, and if it is false, the loop will not run and the computer will move on to the next bit of code after the loop. Typically, you want this condition to test if i is greater than the size of your dataset, which you can input manually, but the best way to do this is to use .length on any array to get its length, which is the number of items inside it.
3. The third part i++ indicates how the value of i should change each time the loop runs. i++ is shorthand for i = i + 1, meaning that each time it runs, add 1 to the value of 1 and make that answer the new value of i. Basically, increment i by 1 each time the loop runs.

The code inside the curly braces runs each time the loop runs. Be warned that it is possible to create infinite loops if you get a loop wrong, though Apps Script does time out if it runs for too long as there is a limited runtime set by Google so it won't actually run forever!

Here's an example of a loop using a 2D array, called myData, which is a range from a spreadsheet that does contain a header row, and how we can do something with each row in turn:

var numberOfRows = myData.length;
for (var i = 1; i < numberOfRows; i++) {
    var singleValue = myData[i][0];
    Logger.log(singleValue);
}

In this example, we get the length of the array numberOfRows, then use that when creating the loop to know when the loop should end. We set the starting value of i to be 1 because we don't want the first array, at position 0, because it is the header row of the data. Insidethe loop, we get a single value from the row of data using the counting variable i to stand in for the position we are at in the main array, and then we log the item of data we have stored in the variable.

The livecoding exercises on this page will help you to get your head around how a for loop works, but once you've used it once, you can just tweak your for loop for different datasets as needed.

You can use numbers, strings, and variables to form the conditions that you can put into if statements - often you will compare a variable to a value or compare two variables. Let's take a look at some examples and what they do:

You can also combine conditions using logical operators (similar to the ones you can find when doing advanced searches on online databases, for example). These allow you to use and and or in your conditions, so you can have two conditions that both need to be true (when using and) or two conditions where only one of them must be true (when using or) in the same if statement.

To combine conditions with an and you need to write && (two ampersands) between them and to combine conditions with an or you need to write || (two pipe symbols) between them. For example (shoeSize > 6 && shoeSize < 10) is true when the value of shoeSize is above 6 but below 10.

Now we have all of the building blocks to create our if condition. We can choose whether to only have the if part, so have code that runs if the condition is true and nothing happens if it is false, or to also have an else part that runs code if the condition is false.

// version with just if
if (name == 'Goose') {
    Logger.log(name + " is Goose");
}

// version with if and else
if (name == 'Goose') {
    Logger.log(name + " is Goose");
} else {
    Logger.log(name + "is not Goose");
}

You can also add other conditions into your if statement using else if so you could have an if, an else if, and then an else at the end.