How do I take first value of sub-array in an array?

You could use the .map() method like so:

var array = [[10,20],[30,40],[50,60]];

var result = array.map(item => item[0]);

console.log(result)

Another way to get the same result;

const arr = [[10,20],[30,40],[50,60]];
const latitudes=[];
for (let i = 0; i < arr.length; i++){
   const arr1 = arr[i];

  latitudes.push(arr1[0]); 

 }

Support in : Chrome(yes) , Firefox(yes), IE5.5+ ,O(yes),Safari(yes)

var arr = [[10,20],[30,40],[50,60]] ;
var i = 0 , res = [] ;

while (i < arr.length ) {
  res.push(arr[i][0]) ;
  i++ ;
}

console.log (res) ;

With ES6 you can also use destructuring & map to get it to this:

var arr = [[10,20],[30,40],[50,60]];

var result = arr.map(([a]) => a)

console.log(result)

You can use array#map with array#destructuring to get the first index item from the individual array.

var locations = [[10,20],[30,40],[50,60]],
    latitudes = locations.map(([latitude]) => latitude);
console.log(latitudes)

You can do it by reduce too.

let obj = [[10,20],[30,40],[50,60]];

const newArr = obj.reduce((nArr, lat) => {
  nArr.push(lat[0]);
  return nArr;
}, []);

console.log(newArr);

You have quite a number of looping mechanisms to choose from. The most straightforward is a map since you expect an array as a result. Using destructuring assignment simplifies the naming, but there is more magic going on and it is ES6-heavy, so keep that in mind if cross-browser compliance is a concern.

let log = console.log
let arr = [[10,20],[30,40],[50,60]]

// Destructuring Assignment
let ex1 = arr.map(([lat,long])=>lat) 
log('ex1:',ex1)

// Non-Destructuring
let ex2 = arr.map(set=>set[0])
log('ex2:',ex2)

// Reduce
let ex3 = arr.reduce((ar,val)=>(ar.push(val[0]),ar),[])
log('ex3:',ex3)

Of course you could get creative and flatten the array and use math to cherry-pick your values:

let arr = [[10,20],[30,40],[50,60]]
let ex4 = arr.flat().filter((v,i)=>!(i%2))
console.log('ex4:',ex4)

Same Using forEach:

let latLongs = [[10, 20], [30, 40], [50, 60]]

let lats = []
latLongs.forEach(
    (latLong) => {
      lats.push(latLong[0])
    }
)

console.log(lats)