Mapping array inside another array using spread operator

Question

I'm testing spread operator inside array to map another array values. Unfortunately, I have come up with weird behavior or I did it wrong. When I return 2 objects using map inside the array, it's only returning the last object. Code below:

const cats = ["Tom", "Ginger", "Angela"];

const array = [
  // {
  //   name: "Ben",
  //   sex: "male"
  // },
  ...cats.map((element, index, array) => {
    return (
      {
        name: element,
        sex: element !== "Angela" ? "male" : "female"
      },
      {
        age: element !== "Angela" ? "20" : "18",
        color:
          element === "Tom"
            ? "black"
            : element === "Ginger"
            ? "orange"
            : "white"
      }
    );
  })
];

console.log(array);

In console:

[{"age":"20","color":"black"},
{"age":"20","color":"orange"},
{"age":"18","color":"white"}]

What I expected:

[{"name": "Tom", "sex": "male"},
{"age":"20","color":"black"},
{"name": "Ginger", "sex": "male"},
{"age":"20","color":"orange"},
{"name": "Angela", "sex": "female"},
{"age":"18","color":"white"}]

Codesandbox here. Is it available to implement it what I expected? Or there are other alternatives?

return makes no sense, you are returning two objects with a coma operator. It will not return both objects, that is not how that operator works. — epascarello
– epascarello, Commented Jan 24, 2023 at 17:24
Also the order looks strange. Why do you expect the first age-color object to come after Tom but then Ginger and Angela following each other. — Yury Tarabanko
– Yury Tarabanko, Commented Jan 24, 2023 at 17:28

epascarello · Accepted Answer · 2023-01-24 17:28:22Z

2

You are returning two objects with a comma. Comma operator will just return the last item. You need to return an array and use flatMap

const cats = ["Tom", "Ginger", "Angela"];
const result = cats.flatMap(x => ([{
  foo: x
}, {
  bar: x
}]));

console.log(result);

answered Jan 24, 2023 at 17:28

epascarello

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Comments

Andrew Parks · Accepted Answer · 2023-01-24 17:38:30Z

0

The problem essentially was that you did return({},{}) instead of return [{},{}].

(a,b) is called a comma expression, and it evaluates the first expression a and then ignores the result, and then only returns the evaluation of the second expression b.

const cats = ["Tom", "Ginger", "Angela"];

const array = cats.map((element) => [{
    name: element,
    sex: element !== "Angela" ? "male" : "female"
  },
  {
    age: element !== "Angela" ? "20" : "18",
    color:
      element === "Tom"
        ? "black"
        : element === "Ginger"
        ? "orange"
        : "white"
  }]
);

console.log(array);

edited Jan 24, 2023 at 17:38

answered Jan 24, 2023 at 17:29

Andrew Parks

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