Sometimes you have to get a single character from a string. For example, if the website knows the user's first name and surname, and at some point, you need to output it as J. Smith, you need to take the initial letter from the first name.
const firstName = 'Tirion';
console.log(firstName[0]); // => 'T'
A number encased in square brackets is the special syntax used for extracting a character from a string. The number is called the index, which represents the position of the character in the string. Indexes start with 0 in almost all programming languages – so to get the first character, you must set the index to 0. The index of the last element is equal to the length of the string minus one:
// String length is 6, so the last index is 5
const firstName = 'Tirion';
console.log(firstName[5]); // => 'n'
// Self-check. What will this code print?
const magic = '\nyou'
console.log(magic[1]); // => ?
You can use variables as well as numbers as an index. Here is an example with the same result (printing T), where the index in square brackets is a constant, not a number:
const firstName = 'Tirion';
const index = 0;
console.log(firstName[index]); // => 'T'
Technically, you can set the index beyond the word length. In our example, these are numbers from 6 and greater. JavaScript does not consider this behavior an error. Addressing a non-existent index will return undefined.
const firstName = 'Tirion';
console.log(firstName[10]); // => undefined
