S4144 should ignore functions with few statements or a single expression

[gabriel-vivas-sonarsource]

Label	Description
Rule	S4144 "Functions should not have identical implementations"
FP Example	* JSX component mapping

Explanation

• It seems that the current implementation is ignoring functions that have only a couple of lines, which seems fair.
• However, this also means the rule is finding an issue for functions that have a few statements in multiple lines. This is particularly visible when you have a single expression that takes multiple lines.
• Particularly in JSX, there can be a single expression that can take many lines. Like when mapping over an Array to create JSX components. See the given example.
• The coupling of using a common function is not worth the little duplication, since in this case there would be little or no logic involved.

Suggestion

• The rule should ignore functions with a single expression
• The rule should ignore functions with few statements
• We should consider ignoring arrow functions that are passed to Array methods like map, filter, find, etc. unless they have many statements.

[francoismora]

I changed the rule to look only at function declaration, method definition, and variable declarations. That's to say, it only looks at functions that are defined in a syntactic scope ignoring all function expressions.

// Function declarations
function foo() {
}

class Foo {
  // Method definitions
  foo() {
  }
}

// Variable declarators:
const foo = () => {
};

The motivation is that it's not practically possible to determine when a function returns just an expression, as there are many different types of expressions. Moreover, there are several ways for a function to return a simple expression (block with a return statement, expression in the body).

Therefore we can expect some true positives to be ignored from now on.