Variables and constants in Apps Script

Variables are used to store values.

To assign a value to a variable, you need to first declare the variable and then assign a value to it.

Declaring a variable

In modern Apps Script, you can declare a variable using either let or const keywords, depending on whether the value needs to change later.

let firstName;    // Use let for variables that will change
const API_KEY = "ABC987XYZ";    // Use const for values that should not change

Since you haven't yet assigned a value to the variable firstName, its value will be undefined.

Assigning a value to a variable

A value can be assigned to a variable by using the equal to (=) operator.

let firstName = "Jack";  // Declare and assign in one step

You can also declare a variable first and assign a value to it later.

let lastName;
lastName = "Lee";

For constants, you must assign a value when you declare them:

const TAX_RATE = 0.05;  // Constants must be initialized when declared

The let and const keywords should only be used when you declare a variable. This only happens once. Once you assign a value to a variable, you can use this value by using the variable's name.

let firstName;
firstName = "Jack";
console.log(firstName);  // Jack

In the above example, the statement console.log(firstName); will output the string "Jack" because that is the value stored in the variable firstName.

Always declare a variable before assigning a value to it

Although Apps Script will allow you to assign a value to a variable that has not yet been declared, please do not do this. This will create something called a global variable and could cause inadvertent bugs in your program.

So, do not assign a value to a variable that has not been declared. Either declare a variable and assign a value to it at the same time OR declare a variable first and then assign a value to it.

// Do not do this
firstName = "Jack";  // this will create a global variable

// This is OK - declare a variable and assign a value to it at the same time
let firstName = "Jack";

// This is also OK - declare a variable first and then assign a value to it
let firstName;  // declaration first
firstName = "Jack";  // followed by assignment

Difference between let and const

The key difference between let and const is that variables declared with let can be reassigned, while variables declared with const cannot be changed after they are initialized:

let age = 25;
age = 26;  // This is fine - let variables can be reassigned

const PI = 3.14159;
PI = 3.14;  // This will cause an error - const variables cannot be reassigned

Assigning a new value to a variable

You can change the value that is assigned to a variable declared with let. Just assign a new value to it using the equal to (=) operator.

let firstName;
console.log(firstName);  // undefined
firstName = "Jack";
console.log(firstName);  // Jack
firstName = "Ryan";
console.log(firstName);  // Ryan
firstName = "Bush";
console.log(firstName);  // Bush

However, you cannot reassign a value to a constant:

const TAX_RATE = 0.05;
TAX_RATE = 0.06;  // This will cause an error

Block scope with let and const

Both let and const are block-scoped, which means they are only accessible within the block they are declared:

if (true) {
  let blockScoped = "I'm only available in this block";
  const ALSO_BLOCK_SCOPED = "Me too!";
}

console.log(blockScoped);        // Error: blockScoped is not defined
console.log(ALSO_BLOCK_SCOPED);  // Error: ALSO_BLOCK_SCOPED is not defined

Using operators with variables

You can also use operators to perform operations on variables. These operators will operate on the values contained in the variables.

const price = 10.0;
const quantity = 20;
let totalAmount = price * quantity;
console.log(totalAmount);  // 200.0

The type of a variable

A variable's type will be the same as the type of the value it contains. For example, if the variable contains a string value then its type is also string. If you haven't assigned any value to a variable, its value will be undefined.

let firstName;
console.log(typeof firstName);  // undefined
firstName = "Jack";
console.log(typeof firstName);  // string

Variable names

Apps Script has certain rules that govern variable names.

• Variable names can only contain letters, numbers, underscores (_) or dollar signs ($).

• Variable names must start with a letter, a dollar sign ($) or an underscore (_). They cannot start with a number.

• Variable names cannot contain spaces.

• You cannot use certain words as variable names. These words are called reserved words. For example, you cannot use let as a variable name. The full list of reserved words can be found here.

Best practices for naming variables and constants

There are also some best practices that you should consider following when naming variables and constants.

• Pick a name that is meaningful. Someone else reading your code should be able to understand the purpose of a variable from its name. For example, the name aaa does not tell us anything about the type of values it might contain. The name firstName tells us a lot about the purpose of the variable and the values it might contain.

• If the name contains multiple words, either use underscores or camel casing to make it easier to read the name. It is very hard to read the name timesheetdueby. It is a lot easier to read time_sheet_due_by or timeSheetDueBy.

• Use CAPS while naming constants so it is easy for someone reading your code to differentiate variables from constants. Also, use underscores to separate multiple words in the name.

const price = 10.0;
const quantity = 20;
const TAX_RATE = 0.05;  // 5% tax rate
let totalAmount = price * quantity * (1 + TAX_RATE);  // order amount + tax

Variable scopes

We'll discuss variable scopes in detail later. For now, it is enough if you know the following:

• Variables can either be local in scope or global in scope.

• Variables declared within a function using let or const are local in scope. This means that their values can only be accessed within the function where they are declared.

• Variables that are declared outside of a function are global in scope. This means that their values can be accessed anywhere in your program (even within functions).

• The block scope of let and const makes them more predictable and reduces bugs compared to var.

if (true) {
  var x = 10;  // Exists outside the if block
  let y = 20;  // Only exists inside the if block
}
console.log(x);  // 10
console.log(y);  // ReferenceError: y is not defined

console.log(a);  // undefined (hoisted but not initialized)
console.log(b);  // ReferenceError: Cannot access 'b' before initialization
var a = 5;
let b = 10;