Checking the digits of a number in JavaScript

Let's have a number:

javascriptlet num = 12345;

Attempting to access the first character of a number will result in unexpected behavior:

javascriptlet num = 12345;

if (num[0] == 1) {
  console.log('+++');
} else {
  console.log('---'); // it will work
}

As you should already know, the problem is that you can access the characters of a string, but not the digits of a number:

javascriptlet num = 12345;
console.log(num[0]); // shows undefined

To solve the problem, let's convert our number to a string:

javascriptlet num = 12345;
let str = String(num);

if (str[0] == 1) {
  console.log('+++'); // it will work
} else {
  console.log('---');
}

It is not necessary to introduce a new variable, you can apply [0] directly to the result of the String function:

javascriptlet num = 12345;

if (String(num)[0] == 1) {
  console.log('right'); // shows 'right'
} else {
  console.log('wrong');
}

Let now we want to check the first digit for the fact that it is equal to 1 or 2. Let's write the corresponding code:

javascriptlet num = 12345;

if (String(num)[0] == 1 || String(num)[0] == 2) {
  console.log('+++');
} else {
  console.log('---');
}

In this case, it turns out that the construct String(num)[0] will be repeated twice. This, firstly, is too long, and secondly, it is not optimal, since we will convert the number into a string twice - the second time it turns out to be redundant, but the program resources are spent on it.

Let's fix the problem:

javascriptlet num = 12345;
let first = String(num)[0];

if (first == 1 || first == 2) {
  console.log('+++');
} else {
  console.log('---');
}