Can I get all 'subjects' values from an object in JS quickly?

marivsh
May 29, 2023
289 views
2 votes
3 Answers

I had an object with this structure:

[
  {
    "event_id": 1,
    "subjects": [
      {
        "id": 12,
        "name": "Chemistry"
      },
      {
        "id": 13,
        "name": "Physics"
      },
      {
        "id": 14,
        "name": "Psychology"
      },
      {
        "id": 16,
        "name": "History"
      }
    ]
  },
  {
    "event_id": 2,
    "subjects": [
      {
        "id": 11,
        "name": "Maths"
      },
      {
        "id": 12,
        "name": "Chemistry"
      },
      {
        "id": 14,
        "name": "Biology"
      },
      {
        "id": 15,
        "name": "Geography"
      }
    ]
  },
  {
    "event_id": 3,
    "subjects": [
      {
        "id": 14,
        "name": "Biology"
      },
      {
        "id": 15,
        "name": "Geography"
      },
      {
        "id": 16,
        "name": "History"
      }
    ]
  }
]

What will be the fastest way to get all possible values of subjects prop in this option? Is this only possible to achieve by looping through each single sub-object and push value to "counting array" if it not already there?

Tags: javascript

Answers

The simplest and most readable way would indeed be to loop through the subjects and add them to the array if they don’t exist.

const data = [
  {
    "event_id": 1,
    "subjects": [
      {
        "id": 12,
        "name": "Chemistry"
      },
      {
        "id": 13,
        "name": "Physics"
      },
      {
        "id": 14,
        "name": "Psychology"
      },
      {
        "id": 16,
        "name": "History"
      }
    ]
  },
  {
    "event_id": 2,
    "subjects": [
      {
        "id": 11,
        "name": "Maths"
      },
      {
        "id": 12,
        "name": "Chemistry"
      },
      {
        "id": 14,
        "name": "Biology"
      },
      {
        "id": 15,
        "name": "Geography"
      }
    ]
  },
  {
    "event_id": 3,
    "subjects": [
      {
        "id": 14,
        "name": "Biology"
      },
      {
        "id": 15,
        "name": "Geography"
      },
      {
        "id": 16,
        "name": "History"
      }
    ]
  }
]

const all = []
for (const { subjects } of data) {
  subjects.forEach((s) => {
    if (all.indexOf(s.name) === -1) {
      all.push(s.name)
    }
  })
}
console.log(all)

Another useful way is to use a Set with spread syntax to deduplicate the values in the array.

const data = [
  {
    "event_id": 1,
    "subjects": [
      {
        "id": 12,
        "name": "Chemistry"
      },
      {
        "id": 13,
        "name": "Physics"
      },
      {
        "id": 14,
        "name": "Psychology"
      },
      {
        "id": 16,
        "name": "History"
      }
    ]
  },
  {
    "event_id": 2,
    "subjects": [
      {
        "id": 11,
        "name": "Maths"
      },
      {
        "id": 12,
        "name": "Chemistry"
      },
      {
        "id": 14,
        "name": "Biology"
      },
      {
        "id": 15,
        "name": "Geography"
      }
    ]
  },
  {
    "event_id": 3,
    "subjects": [
      {
        "id": 14,
        "name": "Biology"
      },
      {
        "id": 15,
        "name": "Geography"
      },
      {
        "id": 16,
        "name": "History"
      }
    ]
  }
]

const all = []
for (const { subjects } of data) {
  subjects.forEach((s) => {
    all.push(s.name)
  })
}
const s = [...new Set(all)]
console.log(s)

just because there’s several ways to do the same things, here’s one with reduce

data.reduce((accumulator, currentValue) => {
    accumulator.push(currentValue.subjects.map(x => x.name));
    return [...new Set(accumulator.flatMap(x => x))];
}, [])


// will output
// ['Chemistry', 'Physics', 'Psychology', 'History', 'Maths', 'Biology', 'Geography']

const data = [
  {
    "event_id": 1,
    "subjects": [
      {
        "id": 12,
        "name": "Chemistry"
      },
      {
        "id": 13,
        "name": "Physics"
      },
      {
        "id": 14,
        "name": "Psychology"
      },
      {
        "id": 16,
        "name": "History"
      }
    ]
  },
  {
    "event_id": 2,
    "subjects": [
      {
        "id": 11,
        "name": "Maths"
      },
      {
        "id": 12,
        "name": "Chemistry"
      },
      {
        "id": 14,
        "name": "Biology"
      },
      {
        "id": 15,
        "name": "Geography"
      }
    ]
  },
  {
    "event_id": 3,
    "subjects": [
      {
        "id": 14,
        "name": "Biology"
      },
      {
        "id": 15,
        "name": "Geography"
      },
      {
        "id": 16,
        "name": "History"
      }
    ]
  }
];

console.log(data.reduce((accumulator, currentValue) => {
    accumulator.push(currentValue.subjects.map(x=>x.name));
    return [...new Set(accumulator.flatMap(x=>x))];
}, []));

const data = [
  {
    "event_id": 1,
    "subjects": [
      {
        "id": 12,
        "name": "Chemistry"
      },
      {
        "id": 13,
        "name": "Physics"
      },
      {
        "id": 14,
        "name": "Psychology"
      },
      {
        "id": 16,
        "name": "History"
      }
    ]
  },
  {
    "event_id": 2,
    "subjects": [
      {
        "id": 11,
        "name": "Maths"
      },
      {
        "id": 12,
        "name": "Chemistry"
      },
      {
        "id": 14,
        "name": "Biology"
      },
      {
        "id": 15,
        "name": "Geography"
      }
    ]
  },
  {
    "event_id": 3,
    "subjects": [
      {
        "id": 14,
        "name": "Biology"
      },
      {
        "id": 15,
        "name": "Geography"
      },
      {
        "id": 16,
        "name": "History"
      }
    ]
  }
];

const allSubjects = data.map(x=> x.subjects).reduce((a,x) => [...a, ...x] ,[]);
const uniqueSubjects = allSubjects.filter((e,i) => allSubjects.findIndex(a => a.id == e.id) == i);

console.log(allSubjects, uniqueSubjects);

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Can I get all ‘subjects’ values from an object in JS quickly?

Answers