So you want to create a function in JavaScript?

You. Have. Options:

1 — Declaration Function

function sum(a, b) { return a + b; }

2 — Expression Function

// Can be named:
(function sum(a, b) { 
    return a + b;
}); 

// Or anonymous (AVOID):
(function(a, b) { 
     return a + b; 
 });
 // Or assigned to a variable
const sum = function sum(a, b) { 
     return a + b; 
  })

3 — Arrow Function

// Single argument, one line return:
    name => name.split(' ');

// Multi arguments, one line return:
    (a, b) => a + b; 

// Single argument, full body:
name => { 
    return name.split(' '); 
}

// Regular:
    (a, b) => { return a + b };
                    

4 — Generator Function

function *sum(a, b) { yield a + b; };

5 — Async Function

async function sum(a, b) { 
    return await a + b; 
};

6 — Constructor Function (AVOID)

new Function(‘a’, ‘b’, ‘return a + b;’);

7 — Exported Function

// Default exports
export default function(a, b) { 
    return a + b; 
};

// Named exports
export function sum(a, b) { 
    return a + b; 
};

8 — Object Property Function

// Regular:
const object = {
  sum: function(a, b) { 
      return a + b; 
  },
};

// Shorthand:
const object = {
  sum(a, b) { 
      return a + b; 
  },
};

9 — Object Dynamic Property Function

const functionName = "sum";

const object = {
  [functionName]: function(a, b) { 
      return a + b; 
  },
};

10 — Object Property Getter/Setter Function

// Regular:
const object = {
  get answer { 
      return 42; 
  },
  set answer(value) { /* do something with value */ },
};

// With defineProperty
const obj = {};

Object.defineProperty(obj, "answer", {
  get() { 
      return 42; 
  },
  set(value) { /* do something with value */ },
});

11 — Object Dynamic Property Getter/Setter Function

const functionName = "answer";

const object = {
  get [functionName]() { 
      return 42; 
  },
  
  set [functionName](value) { /* do something with value */ },
};

12 — Class Method Function

class Compute {
  // Regular:
  sum(a, b) { 
      return a + b; 
  };
};

class Compute {
  // Static:
  static sum(a, b) { 
      return a + b; 
  };
}

13 — Class Property Function

class Compute {
  // Regular:
  sum = function (a, b) { 
      return a + b; 
  };
};

class Compute {
  // Static:
  static sum = function(a, b) { 
  return a + b; 
  };
}

14 — Class Private Function

class Compute {
  // Regular:
  #sum(a, b) {
    return a + b;
  }  
  
  // Static:
  static #sum(a, b) {
    return a + b;
  }
}

I tried to keep the list to only the uniquely different ways to create a function. If you start thinking about combining some of these ways there would be a lot more options.