consecutiveChars.js

// problem in https://twitter.com/Al_Grigor/status/1357028887209902088

const funcReducer = (state, currentChar) => {
    const isNewGroup = currentChar != state.lastChar ? true : false

    switch (isNewGroup) {
        case true:
            state.out = [...state.out, {
                [currentChar]: 1
            }]
            break

        case false:
            const lastItemIdx = state.out.length - 1
            const count = state.out[lastItemIdx][currentChar] + 1

            state.out[lastItemIdx] = {
                [currentChar]: count
            }

            break
    }

    state.lastChar = currentChar

    return state
}

// entry point
const input = "aaaabbbcca".split("")

const result = input.reduce(funcReducer, {
    lastChar: false,
    out: []
})

console.log(result.out)