Js Statements Vs Expressions

Let’s talk about statements and expressions. It’s very important to understand difference between them. I’ve checked several articles on this topic and in a lot of them authors say something like: Ok, we have statements, we have expressions, we can use expressions in place of statements but not in reverse. It is kind of true, but not completely. It’s not like you can apply hammer-wrench hammerandwrench analogy. Like statement is a hammer hammer and expression is a wrench wrench. And as like with heavy enough wrench you can use it as a hammer – you can apply expression in place of a statement. No, it’s not like that. An expression is always part of statement. Even if that statement only consists of that expression.

Statements

When we write programs - we describe the sequences of actions that should be performed to get a desired result. In programming languages those actions are called statements. So every Javascript program basically consists of statements. In javascript statements are separated by semicolons. Important note: You can use just newlines and omit semicolons, because Javascipt puts semicolons in the end of lines automatically, but x this is not recommended. There are cases where it can lead to ambiguity. See the automatic semicolon insertion section of ECMAScript specification. All of those are statements:

let declaredVariable; // variable declaration is a statement
let otherVariable = 0; // even with assignment
function functionCall() { // function declaration is a statement
}
if(true){} // if is statement
2+2; // even this is statement
// Even though it consists from only one expression

As you can see statements just do things. They are kind of language phrases. They can contain smaller parts, for example expressions.

Expressions

Simply speaking an expression returns a value:

2+2
true
true && false
functionCall() // whatever the function returns
declaredVariable // whatever the variable value was
declaredVariable = 'new value' // assignment is an expression

Actually just "foo", {foo: 'bar'}, [1,2,3], or 42 are also expressions, they are called literals (string, object, array and number literal) because they just return their literal value. And even this small expressions can be statements on their own.

2;

This is statement. It’s useless. It doesn’t help, but still. As you can see it consists from only one expression. Statements like this are called expression statements.

Conclusion

Javascript program consists of statements. They are executed one by one, in the same order as they are written. Statements can contain other constructions like expressions, keywords, operators and other stuff. Expressions are parts of statements that return values. So expressions can be used whenever value is expected. Sometimes you can use expression-statements instead of usual statements, if-else is good example:

let foo;
if (bar === 'bazz') {
  foo = bar;
} else {
  foo = null;
}

You can use this expression instead:

let foo = bar === 'bazz' ? bar : null

That’s it, it was short, but I hope it helped to structurize the knowledge.

Dear all, I believe, that this is one of the best explanation regarding this topic!