What is the third parameter thisArg in Array.from method

Question

Array from method can be called with argument items and optional arguments mapfn and thisArg.

Array.from(items, mapfn, thisArg);

The following simple example demonstrate crating new Array using mapfn with multiply logic inside for each items value.

Array.from('123', (item, index) => item * 2); // [2, 4, 6];

What is the third argument? Please give me an example which will showing the case when i should use thisArg.

Answer 1

thisArg is Optional

• Value to use as this when executing mapFn.

a = Array.from('2431', function(item){
  return item*this.multiply;
}, {multiply:2});
console.log(a)
// will return : [4, 8, 6, 2]

and it's identical to:

a = Array.from('2431', (item, index) => item * 2);
console.log(a)
// will return : [4, 8, 6, 2]

https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Array/from

Answer 2

The third parameter is

Value to use as this when executing mapFn.

const x = Array.from('123', function(item) {
  console.log("THIS", this);
  return item * 2;
}, {
  test: "valueOfThis"
});

console.log("Result:", x);

ES6 version

const that = { test: "ValueOfThis" };
const x = Array.from('123', (item) => {
  console.log("THIS", that);
  return item * 2;
});

console.log("Result:", x);