Variable Assignment

In this statment

animal = 'wildebeest';

a variable named animal is set to contain the string 'wildebeest'. However, the = character does NOT represent equality.
Suppose for a second that it does and consider this statement.

   x = x + 1;

If this were asserting equality, then this equation would reduce algebraically to 0=1, which would cause the fabric of the universe to come apart. Not good.

In JavaScript, = is the variable assignment operator. In general, here's how it works.

   variable_name = expression;

Evaluating varaible assignment is a two step process:

   1) Evaluate the expression on the right side.

   2) Store the result into the variable on the left side.

A better symbol to represent variable assignment would be like below, where the stuff on the right is evaluated and stored into the variable on the left.
But ← is not a keyboard character, so programming languages can't use it for variable assignment.

   variable_name ← expression;

Now the following code makes sense.

   var x = 1;  // x is assigned the number 1.

   x = x + 1;  // x is assigned the number 2.

               // The right side evaluates to 2, so 2 is assigned to x, replacing the initial value of 1.

When you assign a new value to a variable that already contains a value, the new value simply replaces the initial value.

The following statement is NOT valid. The quantity on the left must be a variable name, not an expression.

   x + 1 = x;  // ERROR - quantity on left must be a variable name.

The var keyword is used to declare a variable as a storage location. Technically when you declare a variable, it can't literally be empty, so it contains the special value undefined. The first assignment to a new variable is often called initialization, basically assigning the variable it's initial value.

   var num;  // num is an empty variable.

   num = 1;  // num is assigned the number 1.

In JavaScript, you can declare and assign an initial value to a variable in one statement.

   var num = 1;  // Same as above, just in one statement.

You have actually seen it done both ways in the previous examples. Did you notice?