Loop through an array and push it into array of object javascript

Question

So i try to loop an Array and push it into object within this code:

   const past7Days = [...Array(7).keys()].map(index => {
var date = new Date();

  const local = new Date();
  local.setMinutes(date.getMinutes() - date.getTimezoneOffset());
  local.setDate(date.getDate() - index);

  return local.toJSON().slice(0, 10);
});
let data={};
let arr=[]
for (var i = 0; i < past7Days.length; i++) {
    console.log(past7Days[i]);
  return arr.push(data["key"] = past7Days[i]) 
}

what i expected is :

[{key:"date"},{key:"date2"}]

can somebody tellme where did i do wrong here

In an object, you can't have duplicate keys. Try inputting {key:"date",key:"date2"} to the console, you'll see something like {key: 'date2'} as the evaluation result. When you assign to data["key"], you overwrite any older value. — qrsngky
– qrsngky, Commented Nov 15, 2022 at 10:24
Why would you want an object with just one key and that key always being key? Can you explain how that is useful? Why not an array of dates? — trincot
– trincot, Commented Nov 15, 2022 at 10:31
because i am going to use the key:"dates" on my bootstrap vue table as field @trincot thats why i try to make it always being key — JS24
– JS24, Commented Nov 15, 2022 at 10:32

trincot · Accepted Answer · 2022-11-15 10:42:56Z

In your attempt there is only one such object created (data) and the assignments to data[key] just keep overwriting the same object's property. Moreover, that push will not push an object, but the string that is assigned.

You can create each object immediately in your first iteration, with a { key: string-value } object literal and returning that.

Unrelated, but you should not use getTimezoneOffset like that. In boundary cases (like when daylight saving switches on the same day) it can have undesired results. Instead consider that you can convert a date to a string with respect of the current locale's timezone. For instance, the Swedisch locale also uses "YYYY-MM-DD" format (like toJSON), and when used with toLocaleDateString it will use the locale's date:

const past7Days = Array.from({length: 7}, (_, index) => {
    const local = new Date();
    local.setDate(local.getDate() - index);
    return { key: local.toLocaleDateString('en-SE') };
});
console.log(past7Days);

qrsngky · Accepted Answer · 2022-11-15 10:34:14Z

1

Assuming you want exactly one array where the elements are like {"key": <string representation of a date> }, you can try this:

const past7Days = [...Array(7).keys()].map(index => {
var date = new Date();

  const local = new Date();
  local.setMinutes(date.getMinutes() - date.getTimezoneOffset());
  local.setDate(date.getDate() - index);

  return local.toJSON().slice(0, 10);
}); 
let arr=[]
for (var i = 0; i < past7Days.length; i++) {
    // console.log(past7Days[i]);
    arr.push( {key: past7Days[i] } ) 
}
console.log(arr);

answered Nov 15, 2022 at 10:34

qrsngky

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