In JavaScript, are all expressions statements?

Question

So, my understanding is that the distinction between a statement and an expression is that a statement is a line of code that performs some action, whereas an expression produces a value. Further, while it makes sense that not all statements are expressions because not all resolve to a value (as in the cases of if, while, and for), if an expression produces a value, isn't that considered doing something, and therefore also a statement?

Answer 1

Since you asked the question tagged with computer-science as well as javascript I thought I'd point out that JavaScript is different to several other popular languages in this regard.

For example, in Java and C# you cannot use an arithmetic expression as a statement.

public class C {
    public void M() {
        1 + 2;
    }
}

The C# compiler rejects this code with message:

error CS0201: Only assignment, call, increment, decrement, and new object expressions can be used as a statement

A similar error is produced by the Java compiler:

error: not a statement

Such an expression has no observable effect in code, therefore computing it is pointless. The compiler designers chose to flag this as a compile error, presumably to help developers avoid a class of mistakes in their code.

Kotlin produces a warning:

Warning: The expression is unused

The D compiler (dmd) produces:

Error: + has no effect in expression (1 + 2)

GCC 6.3 (both for C and C++) doesn't seem to complain (though it might do if you turn on -Wall or other extended warnings).

Swift also doesn't complain.

So although JavaScript permits such expressions as statements, I would argue that it's a weakness of the language specification rather than a strength. If the return value is not applied anywhere then it's a waste of computation (either at runtime, or at JIT time) and potentially hides a bug, such as a missing return statement.

One final note is that some compilers may be able to elide the expression if it can determine it has no side effects. As an example 1 + 2 + doSomething() cannot be completely removed if doSomething does anything observable (such as print to the screen or update shared state), however the preceding addition operations could be discarded.