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The flat() method creates a new array with all sub-array elements concatenated into it .

It does not alter this (array on which flat is called) but instead returns a shallow copy that contains the same elements as the ones from the original array.

//Syntax
flat()
flat(depth=1)

depth

The depth specifies how deep a nested array should be flattened. Default value of depth is 1.

const arr=[1,2,3,[4,5,[6,7]],8,9];
//when we don't provide depth 
console.log(arr.flat()) // [1,2,3,4,5,[6,7],8,9]  
//in above case since depth was by default 1 ,so it is flattened at 1 level.

//now if we provide depth more than 1.
console.log(arr.flat(depth=2)) // [1,2,3,4,5,6,7,8,9] 
// if we provide depth more than array's length it would not give you error , like in the above case if we give depth more than 2 answer would be same.

Its polyfill

const arr=[1,2,3,[4,5,[6,7]],8,9];
Array.prototype.myFlat= function(depth=1){
//here 'this' refers to the array on which myFlat() is applied
if(!Array.isArray(this)) throw new Error('must be applied on array');
let newArr=[];
this.forEach((item)=>{
if(Array.isArray(item)&&depth>0)
newArr.push(...item.myFlat(depth-1));   //if item is array again apply the same method
else
newArr.push(item);
});

return newArr;
}

console.log(arr.myFlat(2)); // [1,2,3,4,5,6,7,8,9]