Lesson 2: Variables
Two of the most important concepts in programming is the concept of data and code. In the previous lesson, we saw examples of both. console.log() is a code, or instruction, that tells the computer to write something to the sceen, and the text "Hello World!" is the data we wanted to print on the screen.
Variables and Data
In order to keep track of data in our programs, we use something called variables. A variable is a special name that can be assigned a particular value. A value could be a piece of text, a number, or something completely different.
Creating Variables
In TypeScript, we create variables by using the code let:
let my_variable: string = "Hello World!"
The above code creates a variable called “my_variable” and assigns it the string value Hello World!. In TypeScript, we need to specify that the value we want to store in the variable is a string by using the suffix : string, this is called a type declaration. From this point onwards, we can now use the name my_variable to refer to the text Hello World!.
Notice how there are no quotes surrounding my_variable in the above example. This is because we only use double quotes to refer to string data, variables are simply written out as they are named.
An important rule of programming is that we can use variable names in place of the data they refer to anywhere that data could’ve written out directly. For example, we can provide the console.log code with our newly created variable:
let my_variable: string = "Hello World!";
console.log(my_variable);
Notice how that when we feed the console.log code with our variable name, it prints out the value assigned to it, and not the variable name itself.
Changing Variables
Once we have created a variable and assigned it with a value, that doesn’t mean we’re stuck with this variable name being forever tied to that specific value, but we can change the value of a variable whenever we please.
Example 2.1: Re-assigning Variable
let my_variable: string = "Hello World!"
console.log(my_variable);
my_variable = "Hello TSWoW!";
console.log(my_variable);
There are two important things to notice here:
-
Despite that we give
console.logthe same variable twice, it prints out only the value last assigned to the variable. -
The second time we assign a value to the variable, we don’t include the
letor: stringparts, because those are only used when we first create a variable to let the program know that a variable of this name exists, and that we should expect it to contain astringvalue.
Variable Types
In the above examples, we have dealt with the string type, that contains text data.
Another important type in TypeScript is the number type, which can contain integer or decimal numbers:
let my_integer: number = 1007688;
let my_decimal: number = 25.0;
Numbers can also be printed to the screen using console.log:
let my_integer: number = 1007688;
console.log(my_integer)
However, in TypeScript, it is illegal to create a number and then try to assign it a string. The following is not valid code:
let my_integer: number = "Hello World"; // does not work
In the playground link above, notice how the name my_integer has a red squiggly line under it. This is because of the type error that comes right after it.