Javascript Array concat function

Question

case1

let history = [{temp : 123}];
history.concat({temp:1234}); //[{temp :123}, {temp:1234}]

case2

let history = [{temp : 123}];
history.concat([{temp:1234}]): //[{temp :123}, {temp:1234}]

case3

let history = [{temp : 123}];
history.concat([[{temp:1234}]]); //[{temp :123}, [{temp:1234}]]

why concat method returns same output in case1 and case2?

According to me case1 and case3 is expected, but case2 is unexpected.

Answer 1

case1: array.concat(nonArray) -> add the nonArray to the array

case2: array.concat(newArray) -> add every element of newArray to the array

case3: array.concat(twoDArray) -> add every element of twoDArray to the array, but every element of the twoDArray IS ITSELF an array.

case2 actually behaves exactly the same as case3

Answer 2

The concat method creates a new array consisting of the elements in the object on which it is called, followed in order by, for each argument, the elements of that argument (if the argument is an array) or the argument itself (if the argument is not an array). It does not recurse into nested array arguments.

const letters = ['a', 'b', 'c'];
const numbers = [1, 2, 3];

letters.concat(numbers);
// result in ['a', 'b', 'c', 1, 2, 3]

const letters = ['a', 'b', 'c'];

const alphaNumeric = letters.concat(1, [2, 3]);

console.log(alphaNumeric); 
// results in ['a', 'b', 'c', 1, 2, 3]