With this definition, it's easy to see that, if the syntax starts with a language keyword, it's a statement.
BTW, language functions (like 'Array.join()') are expressions because they don't start with a keyword.
mov eax ebx
jump 1235
xor eax eax
Part of the reason fortran was like this is that parsing algorithms were quite primitive. Ideas such as lexing, "tokens", and context-freem grammars, which are standard today, weren't common back then. For example both of the following were valid ways to start a loop in Fortran. Note the lack of whitespace in the second!
do i = 1, 10
doi = 1,10
I think that, in the context of JavaScript, if you want a "quick and dirty" way to differentiate them, the `console.log( /* put thing here */ ) ` idea from the article is the best way to go - if you can validly put the thing there, it's an expression, otherwise it's a statement.
a = alert("hello"); // Assigns a to undefined
a = if (true) alert("hello"); // Uncaught SyntaxError: Unexpected token 'if'A JS program is composed out of statements, which may contain expressions. In particular, in JS, any expression is also valid as a statement. This is not true in all languages (for example, `2` is not a valid statement in Go).
I do agree!
Sorry to bother you but I am a bit confused about this part of your comment:
> `alert("Hello, World!")` or `doSomething()` are usually statements, not expressions;
Can you please clarify what you mean here? Do you mean `alert("Hello, World!")` is an expression? Or do you mean it is not an expression? Or did you mistype?
Your `console.log( /* put thing here */ )` test is a pretty good one and indeed `console.log(alert("Hello, World!"))` passes this test.
function foo() {
alert("Hello, World!");
doSomething();
}
Btw, a better rule of thumb for JS is "if you can add `;` after it and not alter the meaning of the program, it's a statement; otherwise it's an expression".
For example, in C# 7, throws were made into expressions to allow syntax like this:
return a ? b : throw new Exception();