I am using this code to read all the values from each object with the key "category":
const listChapter = datas.map((data) => console.log(data.category))
datas is an array of Objects, and there are 45 objects.
The problem is that when I console.log that, i have many duplicates values, you can see that below:
I want to have one unique value of each category. How can I do that?
Filter duplicates from the data before you iterate through them.
var unique = datas.reduce(function(unique, data) { return unique.indexOf(data.category) === -1 ? unique.concat(data.category) : unique; }, []); You also can do the trick with the Set:
let data = ['Alfa', 'Alfa', 'Beta', 'Gamma', 'Beta', 'Omega', 'Psi', 'Beta', 'Omega']; let arr = [... new Set(data)]; // Or = Array.from(new Set(data)); console.log(arr); Edit: I forgot that you have array of objects. In that case:
let data2 = [ {'category': 'Alfa'}, {'category': 'Alfa'}, {'category': 'Beta'} ] let set = new Set(); data2.forEach((data) => { set.add(data.category); }); let arr2 = [... set]; console.log(arr2); This is minor improvement of Tomek's answer. (I am not able comment yet.)
const arr = [ {'category': 'Alfa'}, {'category': 'Alfa'}, {'category': 'Beta'} ]; const unique = [...new Set(arr.map(item => item.category))]; This approach is faster, because of less iterations:
const listChapter = Object.keys(datas.reduce((ac, el) => { ac[el.category] = true return ac }, {})) arr is the whole array.
for(var i = 0; i < data.length; i++) { if(arr[i] == data.category) { remove item here; } } Here's an alternative one-liner using filter() (just for fun):
console.log(datas.filter((data, i, a) => i === a.findIndex(data2 => data.category === data2.category))); 
listChapterbe?mapover data to get each category into an array and thenfilterthat array and remove any duplicate values, assuming you want an array of strings. Read this answer