If object value is unique in array, do this

Question

If i have an array of objects

[
    {name: 'John', count: 5, isunique: false, occupation: 'carpenter'}, 
    {name: 'Bob', count: 6,  isunique: false, occupation: 'carpenter'}, 
    {name: "John", count: 9, isunique: false, occupation: "barber"}
]

(this array is large), and I want to determine which occupations are unique, and on that condition return a new array where only the boolean isunique is changed like so:

[
    {name: 'John', count: 5, isunique: false, occupation: 'carpenter'}, 
    {name: 'Bob', count: 6,  isunique: false, occupation: 'carpenter'}, 
    {name: "John", count: 9, isunique: true, occupation: "barber"}
]

is this possible to do efficiently in Javascript with a large array? I am completely at a loss here.

Answer 1

One option is:

var count = data.reduce(function(ret, el) {
    ret[el.occupation] = (ret[el.occupation] || 0) + 1;
    return ret;
}, {});

data.forEach(function(el) { el.isunique = count[el.occupation] === 1; });