Learning JavaScript, 3rd Edition

Chapter 5. Expressions and Operators

An expression is a special kind of statement that evaluates to a value. The distinction between an expression statement (which results in a value) and a non-expression statement (which does not) is critical: understanding the difference gives you the tools you need to combine language elements in useful ways.

You can think of a (nonexpression) statement as an instruction, and an expression statement as a request for something. Imagine it’s your first day on the job, and the foreman comes over and says, “Your job is to screw widget A into flange B.” That’s a nonexpression statement: the foreman isn’t asking for the assembled part, merely instructing you to do the assembly. If the foreman instead said, “Screw widget A into flange B, and give it to me for inspection,” that would be equivalent to an expression statement: not only are you being given an instruction, you’re also being asked to return something. You may be thinking that either way something gets made: the assembled part exists whether you set it back on the assembly line or give it to the foreman for inspection. In a programming language, it’s similar: a nonexpression statement usually does produce something, but only an expression statement results in an explicit transfer of the thing that was produced.

Because expressions resolve to a value, we can combine them with other expressions, which in turn can be combined with other expressions, and so on. Nonexpression ...