When looping over an array of objects and adding data to a variable, it always adds an null along with the data

Question

I have a an array of objects, and when I am looping over it add adding a value to a variable, the variable always has an nullin it. I know its a pretty basic question, but stuck on why it adds the null and don't know how to stop it.

(function() {
  let ds = [{
      Name: "A",
      Age: 1
    },
    {
      Name: "B",
      Age: 2
    },
    {
      Name: "C",
      Age: 3
    }
  ];
  let str = null;

  for (let i = 0; i < ds.length; i++) {
    if (ds[i].Name) {
      str += ds[i].Name + ",";
    }
  }
  console.log(str);
})();

It shows as null, A, B, C, and what I want it to show as A,B,C

Does this answer your question? JavaScript String concatenation behavior with null or undefined values — norbitrial
– norbitrial, Commented Jan 10, 2020 at 18:03
Try running null + "42". This should answer your question. Use an empty string initially, or better yet, ds.map(e => e.Name).join(",") since everything has a name or ds.filter(e => e.Name !== undefined).map(e => e.Name).join(",") if you do have undef Name keys. — ggorlen
– ggorlen, Commented Jan 10, 2020 at 18:06

Mara Black · Accepted Answer · 2020-01-10 18:06:24Z

2

The value null is stringified to "null" - ToString . if you initialize it with empty string, will work.

(function() {
  let ds = [{
      Name: "A",
      Age: 1
    },
    {
      Name: "B",
      Age: 2
    },
    {
      Name: "C",
      Age: 3
    }
  ];
  let str = "";

  for (let i = 0; i < ds.length; i++) {
    if (ds[i].Name) {
      str += ds[i].Name + ",";
    }
  }
  console.log(str);
})();

answered Jan 10, 2020 at 18:06

Mara Black

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