Are Primitives in Javascript (eg. 3) Technically Considered Expressions?

Question

In Javascript I know that, for instance, 1 + 2 is an expression ... an expression that returns 3. Since an expression basically means a thing that returns a value, and 3 returns the value 3, it would seem like 3 itself would also be an expression.

However, I've never seen any examples of expressions that were simply primitive values, so I'm curious: is 3, or for that matter 'foo', technically considered to be an expression?

Answer 1

According to the Antlr ECMAScript grammar, an expressionStatement is composed of expressionSequence, which is composed of singleExpression, which can be a literal. Here are the relevant parts:

expressionStatement
 : expressionSequence
 ;

expressionSequence
 : singleExpression ( ',' singleExpression )*
 ;

singleExpression
 : Function Identifier? '(' formalParameterList? ')' '{' functionBody '}' # FunctionExpression
 ... # Cut for brevity
 | literal                                                                # LiteralExpression
 | arrayLiteral                                                           # ArrayLiteralExpression
 | objectLiteral                                                          # ObjectLiteralExpression
 | '(' expressionSequence ')'                                             # ParenthesizedExpression
 ;

literal
 : ( NullLiteral 
   | BooleanLiteral
   | StringLiteral
   | RegularExpressionLiteral
   )
 | numericLiteral
 ;

However, I've never seen any examples of expressions that were simply primitive values

That depends on what you think an expression is.

Answer 2

Yes, they are expressions by themselves, operators and assignments are both optional.

Expressions

An expression is any valid unit of code that resolves to a value.

Conceptually we can consider two types of expressions:

• With Assignment (side effects) : Assign value to a variable. Eg: x=23

• With Evaluation : Those that evaluate and therefore resolve to a value. Eg: 6*6

JavaScript has the following expression categories:

• Arithmetic: Evaluate to a number. eg. 5.2534. (Generally arithmetic operators are used)
• String: Evaluates to a character string, for example, "Foo" or "123". (Generally string operators are used)
• Logical: Evaluates to true or false. ( Generally logical operators are used)
• Left-hand-side expressions: Left values are the destination of an assignment. eg. new or super
• Primary expressions: Basic keywords and general expressions in JS. eg. this

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Based on Mozilla Developer Documentation.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Expressions_and_Operators