Access nested array for all elements in ES6/Javascript

Question

How to access nested array of all elements of an array in ES6? Eg

source = [
    {
       id: '1',
       list: ['a1','a2','a3']  
    },
   {
       id: '2',
       list: ['b1','b2']  
   }
]

i want an output ['a1','a2','a3','b1','b2]

i'm doing this but its returning array with length 1, 0th element ['a1','a2','a3','b1','b2].

source.map(x=>x.list).map(x=>x);