Map the first element of each array of an array of arrays in Javascript

You could return just the first elementn of x, an element of the outer array.

var myData = [["name1", 34.1, 43.1, 55.2], ["name2", 5.3, 23.6, 40.9], ["name3", 43.5, 77.8, 22.4]],
    arrayTitle = myData.map(function(x) {
        return x[0];
    });

console.log(arrayTitle);

Your x itself is an array. So you need not to touch myData again inside.

var arrayTitle = myData.map(function(x) {
    return x[0];
});

or with a traditional loop

myData = [
          ["name1", 34.1, 43.1, 55.2],
          ["name2", 5.3, 23.6, 40.9],
          ["name3", 43.5, 77.8, 22.4]
         ];


var arrayTitle = [];

for(var k in myData)
 arrayTitle.push(myData[k][0]);
 
 console.log(arrayTitle);

With your actual code return [myData[x][0]], you are returning undefined, because x is the iterated item, so it's an array and not an index.

And using the wrapping [] is useless here, it's a destructing syntax, we don't need it here.

You should only return item[0] in each iteration:

var res = myData.map(function(arr) {
  return arr[0];
});

Demo:

myData = [
  ["name1", 34.1, 43.1, 55.2],
  ["name2", 5.3, 23.6, 40.9],
  ["name3", 43.5, 77.8, 22.4]
];


var res = myData.map(function(arr) {
  return [arr[x][0]];
});
console.log(res);

This could be the shortest answer:

var res = myData.map(a => a[0])