Get an array from an object using a map in javascript. but getting an array with undefined

Question

I use a map to get a subset of the array.

var mappingobj = {"custom":[{"head":"202","sub":["302","303"]},{"head":"203","sub":["102"]}],"spec":[]};

var subids = mappingobj.custom.map(function(o, i) {
      if(o.head==202){
       return o.sub;
      }
});

console.log(subids);

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I need to get only ["302", "303"]. but I get the output as [Array [2], undefined].

Answer 1

The main idea is that the map should get a specific value from the array. if you do not return anything on the map by default, undefined will return and you will get undefined. Use the filter to obtain the desired object and use the map to obtain the desired property of this object

var mappingobj = {"custom":[{"head":"202","sub":["302","303"]},{"head":"203","sub":["102"]}],"spec":[]};

var subids = mappingobj.custom.filter(function(o, i) {
      if(o.head==202 && o.head){
       return o;
      }
}).reduce((acc,elem)=>{acc.push(...elem.sub); return acc;},[]);

console.log(subids);

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Answer 2

Hi, you can try this.

var mappingobj = {
  "custom": [{
    "head": "202",
    "sub": ["302", "303"]
  }, {
    "head": "203",
    "sub": ["102"]
  }],
  "spec": []
};

var subids = mappingobj.custom.filter(function(o, i) {
  if (o.head == 202) {
    return o.sub;
  }
});

console.log(subids);

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Hope this helps you.

Answer 3

indexOf of head, sub of mapping.custom

var mappingobj = {"custom":[{"head":"202","sub":["302","303"]},{"head":"203","sub":["102"]}],"spec":[]};
var obj = mappingobj.custom;
var index = obj.map(function (e) { return e.head; }).indexOf("202");
console.log(obj[index].sub);

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