How to push null in specify array

Just push until it's 3 long.

var arr = [301];
while(arr.length<3){
    arr.push(null);
}
console.log(arr); // Array(3) [ 301, null, null ]

There are a few ways to do this, here's one way

const obj = {
    08.02: [],
    09.02: [301],
    10.02: [null, null, null],
    11.02: [839, 695, 410],
    12.02: [null, null, null],
    13.02: [839, 679, 407],
    14.02: [null, null, null],
    15.02: [null, null, null],
    16.02: [403, null],
    17.02: [840, 690, 410]
}

for (let key in obj) {
   while (obj[key].length < 3) {
     obj[key].push(null)
   }
}

One way of doing this is to have a function like this:

const fillUpToN = <A>(as: A[], n: number = 3): A[] =>
  as.length < n ? as.concat(Array(n - as.length).fill(null)) : as;

const a = [1, 2]
const b = [1]
fillUpToN(a) // [1, 2, null]
fillUpToN(b) // [1, null, null]

This looks at the argument's length to determine whether we should concatenate anything at the end, and leaves it untouched if it's already long enough.

A default parameter can be used to generalise the function even more.

fillUpToN(b, 5) // [1, null, null, null, null]

First of all, JavaScript does not have associative arrays. So perhaps what you're using is an object. If that's the case, then what you could do is the following:

const obj = {
  "08.02": [],
  "09.02": [301],
  "10.02": [null, null, null],
  "11.02": [839, 695, 410],
  "12.02": [null, null, null],
  "13.02": [839, 679, 407],
  "14.02": [null, null, null],
  "15.02": [null, null, null],
  "16.02": [403, null],
  "17.02": [840, 690, 410]
};

const result = Object.keys(obj).map(key => {
  const arr = obj[key];
  if (arr.length === 3) {
    return { key, items: arr };
  }
  const nulls = Array.from({ length: 3 - arr.length }).map(() => null);

  return {
    key,
    items: [...arr, ...nulls]
  };
}).reduce((accumulator, item) => {
  return {
    ...accumulator,
    [item.key]: item.items
  }
}, {});

console.log(result)

Alternatively, if you just have an array with sub arrays in it, you could do something like the following:

const arr = [
  [],
  [301],
  [null, null, null],
  [839, 695, 410],
  [null, null, null],
  [839, 679, 407],
  [null, null, null],
  [null, null, null],
  [403, null],
  [840, 690, 410]
];

const result = arr.map(subArray => {
  if (subArray.length === 3) {
    return subArray;
  }
  const nulls = Array.from({ length: 3 - subArray.length }).map(() => null);

  return [...subArray, ...nulls];
});

console.log(result);