CODEWITHSUNDEEP
javascript
vantage5353
vantage5353

Reputation: 565

Mozilla states expressions are also statements. Why?

Looking at these articles from Mozilla's JavaScript guide:

Expressions

Statements

expressions are also considered assignment statements. In fact, in the second article one can read "any expression is also a statement". Being acquainted with other programming languages, I thought that expressions are always values, but they never cause side effects like statements would do. In other words, 7, 7 + 8, "string", etc., are expressions, because they don't change the state, but a = 7 is a statement, since a variable has now been defined (i.e. a state has changed).

Why would Mozilla not differentiate between the two in JS?

Upvotes: 2

Views: 101

Answers (3)

dot slash hack
dot slash hack

Reputation: 568

An expression is a code fragment that returns some value, Expression (Computer Science):

3; // 3
{}; // Object
func(); // whatever func returns, or undefined if not specified

You can combine expressions into one compound expression:

3 + 7; // 10
{}, []; // Array. Comma operator always returns result of right-most expression

A statement is the smallest valid code fragment that can be compiled or interpreted, Statement (Computer Science):

5; // valid js

You can also combine statements into compound statements:

check || func(); // valid js
{
    4 + 9;
    "block statement";
}

In the Mozilla documentation, a statement refers to any (compound) statement that is explicitly or implicitly terminated by semi-colon (;).

[,,[],[,[,,],,]]; // Array declaration whose reference is returned (and ignored)
// Multi-dimensional array with empty (undefined) elements

In some programming languages the above example doesn't compile or doesn't get interpreted. Other languages might not allow for the result of an expression not to be catched.

Javascript is very expressive, which is why every expression counts as a valid statement. Some statements are not expressions, like break, return, while, etc. They don't return any value, but they control the program execution flow.

Upvotes: 1

Sebastian
Sebastian

Reputation: 8005

Mozilla does differentiate between the two, or rather the Javascript syntax does.

The only slightly "special" about Javascript is the following:

"any expression is also a statement",

which means that at places where a statement is required in the syntax, an expression can be used directly (but not the other way around). E.g. the following is valid Javascript but invalid in many other similar languages:

if (true) "asfd"

or

foo = function(){
  if (5) { 
    "some text here that won't do anything";
    return true;
    42; // always good to have that one here!
  }
}

whereas statements cannot be used as expressions:

a = (if (true) 5)   // does not work "unexpected token 'if'"

They used that "feature" for the strict mode specification without introducing a new keyword or syntax - if you add the expression "use strict" as the first statement in a function body, Javascript is executed in strict mode in supporting browsers.

While expressions evaluate to a value, usually, statements do not. Most statements alter control flow, expressions usually don't (although one could argue that an expression that results in an exception being thrown alters control flow, too). In Javascript expressions form a subset of all statements.

Upvotes: 0

Daniel Rikowski
Daniel Rikowski

Reputation: 72514

I believe you are taking the terms "expression" and "statement" too literally. "Expressions not changing any state" is a very tough requirement for a programming language.

A thought experiment: In 7 + 8 substitute 8 with a function call to 

var globalVar = 0;
function my8() { 
  globalVar = globalVar + 1; 
  return 8;
}

Is 7 + my8() a statement or an expression? There is no obvious state change happing here, but still my8 performs a state change. Using the "no side-effects" definition it would be impossible to decide if 7 + my8() is a statement or an expression without analyzing the code of the my8 function. Of course it would be possible to simply prohibit any state change as part of a function call, but that is not the way of JavaScript.

In my experience most languages define "everything which returns a value" as an expression and a statement, everything else as just a statement.

To answer your question "Why would Mozilla not differentiate between the two in JS?":

I think they do, but not in the manner you expected. To consider "everything which returns a value" an expression seems to be the most practical approach.

Also there is no contradiction between a chunk of code being a statement and an expression at the same time. That is simply how Javascript and many other languages work. Of course it is always possible to draw a more strict line between those two.

Examples:

Assignments return values, so this is possible:

 a = b = c = 1;

It can be written in the more obvious form:

 a = (b = (c = 1));

Because of that an assignment is considered an expression (and also a statement).

On the other hand:

if (true) { };

does not return a value (in Javascript!) and therefore is no expression (but still a statement).

Upvotes: 5

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