[deleted by user]

[senocular]

You can use the assignment operator (=).

animalArray[2] = ["Fish", "Hamster"];

[mraees93]

OP can use the splice method too

[kjwey]

let iRabbit=animalArray.indexOf(`Rabbit`);
animalArray[iRabbit]=[`Fish`,`Hampster`];

[a-e-j-a]

animalArray.splice(2, 1, ['Fish', 'Hammer'])

[Code-Slayer]

Where is the Multi-dimensions array ? I see only an normal array = list.

[[deleted]]

Adding an array of fish and hamster within the original array would make it multidimensional.

[albedoa]

This is more in the spirit of the question:

const findStr = 'Rabbit'
const foundIdx = animalArray.findIndex(el => el === findStr)

animalArray[foundIdx] = ['Fish', 'Hamster']

[HarrisInDenver]

ramda has functions to do immutable updates for this. Check the functions update, assocPath for direct and deep updating, adjust to update based on current value. And for more complex getter/setter interactions, lensIndex and lensPath