Count the duplicates in a string array using React JS - arrays

Question

Following is a code I implemented to create a bar chart using chart js in React app. Here it creates a bar chart with all the data in an array. But, I want to change this code only to give the output in the x-axis - destination, y-axis - no. of occurrence of this destination since it has many repeated destinations.
I searched methods to this but I couldn't get a correct solution.
Can anyone help me to do this?
const dataArrayY4 = [];
res.data.map(item => {
dataArrayY4.push(item.time)
})
const dataArrayX4 = []
res.data.map(item => {
dataArrayX4.push(item.destination)
})
this.setState({
data4: dataArrayY4,
labels4: dataArrayX4,
});

Answer 1

This could be done as follows:
const res = {
data: [
{ time: 1, destination: 'A'},
{ time: 3, destination: 'A'},
{ time: 2, destination: 'B'}
]
};
let tmp4 = [];
res.data.map((o, i) => {
const existing = tmp4.find(e => e.destination == o.destination);
if (existing) {
existing.time += o.time;
} else {
tmp4.push({time: o.time, destination: o.destination});
}
})
this.setState({
data4: tmp.map(o => o.time);
labels4: tmp.map(o => o.destination);
});
Above code could further be optimized by using Array.reduce() instead of Array.map().

Answer 2

I would make the code more efficient. Instead of dataArrayY4 being an array, I would make it an object that has a key of value and the number of occurrence of each value. This way, you can count all the number of occurrences of the all items in res.data
const dataArrayY4 = {};
res.data.map(item => {
dataArrayY4[item.destination] = (dataArrayY4[item.destination] || 0) + 1
})
const dataArrayX4 = []
res.data.forEach(item => {
dataArrayX4.push(item.destination)
})
this.setState({
data4: dataArrayY4,
labels4: dataArrayX4,
});
Then if you want to look for the occurrence of a particular value you
use this eg. Sri Lanka
this.state.data4['Sri Lanka']