Why i cannot push selectively?

You can selectively add items to an array, by manually adding them in the order you wish to add them. For instance:

const fruit = ['Apples' , 'Bananas', 'Oranges'];

const newFruit = ['Grapes', 'Pineapple', 'Melon'];

// I could either add a load of fruit at once, or only add specific fruit.

function addNewFruit (fruit) {
  fruit.push(...newFruit);   // This will iterate and push items in the newFruit array to the fruit array.

  // RESULT:  ['Apples' , 'Bananas', 'Oranges', 'Grapes', 'Pineapple', 'Melon']
}

function addSomeNewFruit (fruit) {
  fruit.push(newFruit[0]); // Add the first item in the newFruit array to the fruit array
  // RESULT: ['Apples' , 'Bananas', 'Oranges', 'Grapes']

  fruit.push(newFruit[2]); // Add the third item in thew newFruit array to the fruit array 
 // RESULT:  ['Apples' , 'Bananas', 'Oranges', 'Grapes', 'Melon']
}

This is fine for small arrays of items, but imagine if you had tens or hundreds of items! It's not common to want to add only certain items within an array to another one, but you could do it if you specify the index of the item you wish to add, rather than the entire array.

It looks as if your code allows you to "selectively" push a new teacher to the existing teachers array of objects. If you are wanting to push the entire, new array of objects newTeachers, then you'd have to iterate over each entry in the new array. Without iterating over each object in the new array, it will push the entire array as an entry into teachers, which will end up looking like this:

[ { name: 'Ashley', topicArea: 'Javascript' },
  [ { name: 'James', topicArea: 'Javascript' },
    { name: 'Treasure', topicArea: 'Javascript' } ] ]

To iterate over each entry in newTeachers and insert into the existing teachers array, you can use the Array.forEach() method found here. Implementation of this method would look like this:

const teachers = [ { name: 'Ashley', topicArea: 'Javascript' } ]; 
let newTeachers = [ { name: 'James', topicArea: 'Javascript' }, { name: 'Treasure', topicArea: 'Javascript' } ];

newTeachers.forEach(i => {
  teachers.push(i);
});

This will insert each object (in this case, teacher) into its own entry in the existing teachers array.

[ { name: 'Ashley', topicArea: 'Javascript' },
  { name: 'James', topicArea: 'Javascript' },
  { name: 'Treasure', topicArea: 'Javascript' } ]

Sorry, if i explain wrong. If we have a lot of "newTeachers", but i need to push only second and fourth, in one line of code. https://w.trhou.se/ft7mytoxu9 see pls line 34