How can I made new array from firsts elements of arrays from an array

You could just use a simple map and destructure the first element:

const arr = [["1", 2],["3", 2],["6", 2]]
console.log(arr.map(([e]) => e))

The ([e]) part of that before the => is destructuring the parameter using array destructuring. It means that for each subarray passed to the map callback, e receives the value of the first element of the subarray. In ES5 and earlier, the ([e]) => e arrow function would be function(entry) { return entry[0]; }

Still, if you still don't understand the concept, prefer efficiency, or just want to go back to basics, you could use the trusty for loop, making sure to push only the first element of each subarray:

const arr = [["1", 2],["3", 2],["6", 2]]

const output = []
for (let i = 0; i < arr.length; i++) {
  output.push(arr[i][0])
}

console.log(output)

Try this:

let arr = [["1", 2], ["3", 2], ["6", 2]];
let res = arr.map(val => {
    return val[0]
})
console.log(res);

You can use Array.prototype.map() to crate a new array with the item from the first index:

var arr = [["1",2],["3",2],["6",2]]
var newArray = arr.map(i => i[0]);
console.log(newArray);