Extending Array.prototype crashes

Question

I was trying to solve the following problem which I got on a blog but the program crashes. What could be the reason? and is there any means of solving it? I have read warnings not to extend builtin objects, if that's the case, what could be the reason associated with this specific example.

const a = [1, 2, 3, 4, 5];

//this is what I tried
Array.prototype.multiply = function() {
  for (m of this) this.push(m * m);
}

a.multiply(); //this should not be changed
console.log(a); // [1, 2, 3, 4, 5, 1, 4, 9, 16, 25] (expected output)

Code Maniac · Accepted Answer · 2019-10-05 09:37:39Z

3

When you push the value to same array during loop, you end up in infinite loop, create a temp array push value to it, in the end add it to this

const a = [1, 2, 3, 4, 5];

//this is the what I tried
Array.prototype.multiply = function() {
  let newArr = []
  for (const m of this) {
    newArr.push(m * m)
  }
  this.push(...newArr)
}

a.multiply();
console.log(a);

That being said you should not override the prototype simply use a function and pass the parameters

const a = [1, 2, 3, 4, 5];

function multiply(arr) {
  return [...arr, ...arr.map(a => a * a)]
}

console.log(multiply(a));

edited Oct 5, 2019 at 9:37

answered Oct 5, 2019 at 2:54

Code Maniac

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2 Comments

Addis Over a year ago

As for the second method, I know there are much simple ways of doing it but "a.multiply();" is not part of my solution, it was part of the original question.

Code Maniac Over a year ago

@Addis always happy to help :) yeah the second one is to mention that we should be taking simple paths unless we are forced to take the odd one

Nick · Accepted Answer · 2019-10-05 03:05:58Z

1

Pushing a new value into an array in the middle of a for ... of loop creates an infinite loop as the loop includes the new values. You can use forEach instead as that ignores the new values added:

const a = [1, 2, 3, 4, 5];

Array.prototype.multiply = function() {
    this.forEach(v => this.push(v*v));
}

a.multiply(); //this should not be changed
console.log(a); // [1, 2, 3, 4, 5, 1, 4, 9, 16, 25] (expected output)

edited Oct 5, 2019 at 3:05

answered Oct 5, 2019 at 2:57

Nick

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