When does the Shocking Grasp spell actually start preventing the target from taking reactions? [duplicate]

Question

I have now asked my "real" question (one whose answer effectively answers this question) here ("Are getting hit and taking damage simultaneous events").

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The shocking grasp spell's description states:

On a hit, the target takes 1d8 lightning damage, and it can't take reactions until the start of its next turn.

It is unclear to me when the reaction prevention actually occurs (on being hit or on taking damage).
I have found this Q/A ("Would Shocking Grasp prevent the target from casting Absorb Elements?") where a comment from @DavidCoffron says:

[shocking grasp] interrupts the damage taking (which is the trigger), not the hit (which causes the reaction-block)

And then there is this Q/A ("Can the Shield spell be used against Shocking Grasp?") where the following sentence exists in @chaoticgeek's answer:

In addition you take damage and can't take reactions together so if you can prevent the first you have to prevent the other since it is a chain linked by 'and' in this situation...

[T]he reaction prevention only comes with the damage that is dealt

Does the 'and' in shocking grasp really link the clauses enough to make them simultaneous? Are the hit and the damage happening at the same time?
Does shocking grasp prevents the target from taking reactions once the spell hits them or once the spell damages them?

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I do not believe that this Q/A ("When Shocking Grasp hits, can it trigger Wrath of the Storm reaction?") is a duplicate of mine because I am specifically wondering when you are prevented from taking reactions, which (as the quotes above show) does not seem to be entirely agreed upon.

Answer 1

The reaction prevention happens in response to the "on a hit" trigger.

Shocking Grasp has two distinct effects when it hits: it deals 1d8 lightning damage and also prevents the target taking reactions until the start of its next turn. Two separate things happening in response to the same trigger.

As stated in the DMG:

If a reaction has no timing specified, or the timing is unclear, the reaction occurs after its trigger finishes, as in the Ready action.

Finishing the "on a hit" trigger requires both dealing the damage and preventing reactions, so normally a reaction with no specific timing won't happen until after the target can't take reactions any more.

If the reaction specifies that it can be used to negate its trigger, as Shield does, it could be used before the reaction prevention takes place (important, because Shield's effect may prevent Shocking Grasp from hitting in the first place).

Answer 2

The hit, damage, and inability to react all happen simultaneously...

The damage that Shocking Grasp causes occurs with the hit (simultaneously with the hit). The "and" in the spell's description indicates that the secondary effect happen at the same time, which removes the targets ability to react.

...unless the reaction used specifically states otherwise

The spell Shield has a casting time of 1 reaction,

which you take when you are hit by an attack or targeted by the magic missile spell

The spell description is as follows:

An invisible barrier of magical force appears and protects you. Until the start of your next turn, you have a +5 bonus to AC, including against the triggering attack, and you take no damage from magic missile.

In this case, you can use your reaction to defend against Shocking Grasp, because the spell specifically says you can. If the reaction's description does not grant this timing, then you cannot use your reaction against the damage of Shocking Grasp.

There is also a section in the DMG called "Adjudicating Reaction Timing." It says:

Use this rule of thumb: follow whatever timing is specified in the reaction’s description. For example, the opportunity attack and the shield spell are clear about the fact that they can interrupt their triggers. If a reaction has no timing specified, or the timing is unclear, the reaction occurs after its trigger finishes, as in the Ready action.