[RPG] Using Extra Attack feature with two-handed weapon and additional thrown weapon

Taking one hand off a weapon does not use up any part of the action economy

There is a list of things you can do with your object interaction on your turn and a number of them involve picking things up, drawing things, or putting things away into specific places. Conspicuously missing from this list is anything about simply dropping an item nor anything about changing your grip on an item. This is especially notable because the list includes both opening and closing a door, yet only picking up a weapon, not dropping one. Thus we can conclude that dropping an item and changing your grip on an item do not use up your object interaction, nor any other part of your action economy.

Others generally agree that removing one hand from a weapon can be done for free (quotes are from the most upvoted answers):

• Can you cast spells with a Somatic component if you're holding a two-handed weapon?

[...] Taking your hand off the weapon should not require any action expenditure - you are just letting go of it, same as if you dropped it. [...]

• Can you drop one hand from a two-handed weapon to draw and use a single-handed weapon next turn?

[...]Release one hand from two-handed weapon (non-action) [...]

• Action economies, two-handed weapons, and spells with Somatic components

[...] Taking your hand off it in the first place should not require any action expenditure - you are just letting go of something, same as if you dropped it. [...]

Lead Rules Designer Jeremy Crawford also stated in a tweet that this is the case:

Q. what are the rules on dropping weapons? People are dropping weapons to circumvent only having one 'Interaction with Object'

A. The intent is that letting go of something requires no appreciable effort. But picking it up does.

Putting a hand onto a weapon might require your object interaction

I've written about this before and will mostly repeat from my other answers found under the following:

• Does switching between using a versatile weapon one-handed to two-handed cost anything with regards to the action economy?
• Does Improved Pact Weapon allow casting a spell with somatic and material components using a Heavy-crossbow as the focus?

We don't know very many things about this scenario but we do know that drawing a weapon (going from 0-handed to not 0-handed) uses your object interaction. We also know dropping a weapon and then picking it up would require you object interaction (going from 1-handed to 0-handed to 2-handed). But I wasn't able to find anything on a direct change of handedness. Thus it would ultimately be up to the GM whether you can go from holding something in 1 hand to holding it in 2 for free.

I have personally ruled that this can be done for free and have not seen this creating any problems in my own games.

There is also this series of Crawford tweets that could be used to support this stance:

Q. Help resolve a debate. By RAI should a caster be able to prop a 2h weap against his shoulder in order to cast?

A. Yeah, I would allow it too. A two-handed weapon needs two hands to be used, but not necessarily two to be carried.

Q. He's basically saying that switching between 1 or 2 hands is a non-action.

A. That's correct.

This somewhat supports that going from 1-handed to 2-handed is a non-action, but it's not rules-based, nor is it a direct statement that that truly is meant to be the case.

That said, the answer I used earlier about Somatic components states the following:

[...] You can then use your free object interaction to restore your grip after casting. [...]

There are no quoted rules to support this stance, but the answer does have a score of 57.

Regardless, you technically cannot use your bonus action on the attack granted by the Polearm Master feat because you attacked with a javelin

A similar situation is described in the following questions:

• Can you shove (Attack Action), attack (Extra Attack) and use Polearm Master's bonus action attack?
• Can I use Polearm master's bonus attack if attack with a quarterstaff and an unarmed strike(Extra attack)?

The crux of the argument here depends on the following phrasing in the Polearm Master feat (emphasis mine):

When you take the Attack action and attack with only a glaive, halberd, quarterstaff, or spear you can use a bonus action to make a melee attack with the opposite end of the weapon. [...]

You can use this bonus action if your Attack action consists only of attacks using glaives, halberds, quarterstaves and spears; however, in your example you are throwing a javelin and thus your Attack action does not consist solely of the required attacks and thus you cannot take the bonus action granted by the feat.

That said, this is a rather small thing to draw such a distinction on and I believe a GM would be well within their rights to rule otherwise.

A summary of the action-economy requirements of your proposed scenario

1. Start turn with Halberd in both hands

   • This just happens

2. Let go of Halberd with one hand completely for free

   • This is how myself and others would rule

3. Draw Javelin from storage

   • This uses your one free object interaction

4. Throw said Javelin at one target out of melee range

   • This is part of the Attack action

5. Replace second hand onto halberd

   • This might require a second object interaction and thus would not be possible;¹ at my own tables this can be done for free

6. Attack second target that is within melee range

   • This is part of the Attack action

7. Use Bonus Action to do whatever I want, including possibly butt-end attack with Halberd from Polearm Master feat

   • This is technically not possible because your Attack action had an attack using a javelin; at my own tables I would allow using Polearm Master's bonus action attack

¹ Unless your character has the Thrown Weapon Fighting Style from Tasha's Cauldron of Everything, which would allow them to draw the javelin from step 3 for free and use their object interaction in step 5 to put their second hand onto the halberd.