Variables and Value Assignments
Computers can not only compute numbers, but also remember a lot of information. Variables are the computer's agents to store information. They act like containers that can hold values including numbers, strings and other types of data.
Simply put, a variable is a name that refers to a value, it is a "storage" for data.
The name must contain only letters, digits, symbols
The first character must not be a digit
Variable names are case sensitive, so
Aare two different variables.
Reserved words cannot be used as names
For instance, these are valid variable names:
Variables should be defined (or declared) before they are used.
let keyword to declare it.
let num123; let aVeryLongName;
Giving a variable a value is called assigning value, which is done in an assignment statement.
= is an "assignment" operator, not an "equal to" operator. This is different from algebra.
Now we can put some data into a variable by using the assignment operator
let x; x = 1058;
After declaration, we put the variable on the left side of the equal sign, on the right side is the value you want the variable to be assigned to, such as a number or a string. We can access it using the variable name. Let's display it in the console window:
the variable content is showed in the console:
we can also merge the variable declaration and assignment into a single line:
let x = 1058;
Let's use CodeCraft to demo some value assignments:
// variable declaration and simple value assignments let a = 10; let b = 4; // build columns of height a(box_blue: 2), height b(box_orange: 10) column_m(1, -20, a, 2); // blue, height 10 column_m(3, -20, b, 10); // orange, height 4 // use expression to assign values, let c = 2 + 3; column_m(5, -20, c, 6); // green, height 5 // assign sum of (a+b+c) to variable 'total' let total = a + b + c; column_m(7, -20, total, 13); // red, height total=10+4+2+3=19
Reassigning values to variables
When the value of a variable is changed, the old data is removed from the variable and replaced with the new data.
We can declare two variables and copy data from one into the other.
// a series of assignments let f, d; f = 3; d = f; f = 6; // display the results as columns with height of f and d. column_m(16,-20, f, 78); // white column height f (3 or 6? correct: 6) column_m(18, -20, d, 1); // black column height d (3 or 6? correct: 3)
As we continue the lesson, you'll see JS has many other types of data. We can assign them to variables just like strings or integers.