How to catch array index out of bound in JavaScript?

Question

I want to catch an error when access outside of array's bounds happens but it doesn't seem to throw an error.

let arr = [2,4,5];
let test = arr[3];
console.log(test);

I know that I can test for undefined and throw an error

if (arr[3] === undefined) throw new Error();

but how come I can't just do try and catch it like the following.

let arr = [2,4,5];
try {
      let tmp = arr[3];
    } catch(e) {
      lastIndex = findLastIndex(arr, high / 2, high);
      break;
    }

Accessing an undefined property in JavaScript does not cause a runtime error. It's just the way the language works. — Pointy
– Pointy, Commented Jul 4, 2019 at 21:50
Yes, JS doesn't throw an error when accessing a non-existing property. If you want to handle it, do it anticipatory: if (i < arr.length) tmp = arr[i] else …. — Bergi
– Bergi, Commented Jul 4, 2019 at 21:55

Snow · Accepted Answer · 2019-07-04 21:50:05Z

9

As you can see, accessing a non-existent array index does not throw an error - the value "accessed" will simply be undefined. Ordinary objects work the same way:

const obj = {};
// No error:
const val = obj.foo;

console.log(val);

But errors generally should not be used for control flow - errors should handle exceptional cases. For what you're doing, I'd use if/else instead, eg

const arr = [0, 1, 2];
if (4 in arr) {
  const val = arr[4];
  // ...
} else {
  console.log('4 is not in the arr');
}

answered Jul 4, 2019 at 21:50

Snow

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