which one?

a.js

function addSession (subject, calEvent) {
    let index = getIndex(this.sessions, (session) => session.subject === subject.id)

    if (index === -1) {
        this.sessions.push({
            subject: subject.id,
            hours: 1,
            events: calEvent ? [ calEvent ] : []
        })
    } else {
        this.sessions[index].hours += 1
        if (calEvent) {
            let calEventIndex = getIndex(this.sessions[index], (ev) => ev === calEvent.id)
            if (calEventIndex === -1) {
                this.sessions[index].events.push(calEvent)
            } else {
                throw new Error('That calendar event has already been added to the cart.')
            }
        }
    }

    return this
}

b.js

function addSession (subject, calEvent) {
    let index = getIndex(this.sessions, (session) => session.subject === subject.id)

    if (-1 === index) {
        this.sessions.push({ subject: subject.id, hours: 0, events: [] })
        index = 0
    }
    
    this.sessions[index].hours += 1
    
    if (calEvent) {
        let calEventIndex = getIndex(this.sessions[index], (ev) => ev === calEvent.id)
        if (calEventIndex === -1) {
            this.sessions[index].events.push(calEvent)
        } else {
            throw new Error('That calendar event has already been added to the cart.')
        }
    }
    
    return this
}