Concatenation is the process of combining multiple strings to yield one new, bigger string. You may want to construct a message for the user, tailored to be appropriate for varying conditions. In a sense, by now, you're already familiar with what an expression is, even though I haven't explicitly defined it yet. I wanted to keep some suspense in this tale, after all. An expression, in programming circles, is any phrase of program code that represents one evaluation, result, or assignment.
Every little segment covered in examples thus far is an expression.
There is one sort of JavaScript that's worth special note, though. A conditional expression is basically a shorthand way to assign a value to a variable based on the results of a certain condition. A common use for this sort of conditional expression is in an assignment operation, in which a variable is assigned the result of the above expression. Arithmetic operators are used for basic math calculations, as well as for incrementing and decrementing numeric variables. Comparison operators are used to evaluate specified cases, such as whether one variable has a greater value than another, whether they are equal, and so forth.