How strings are made in JavaScript?

For the dumb computers to make sense of JavaScript strings, they must be encoded in some way. JavaScript does it using UTF-16.

What is UTF-16? It stands for 16-bit Unicode Transformation Format. But what's that mean? It means a string in JavaScript is a sequence of 16 bit code units. You might be asking what's a code unit?

Well a code unit is a number to encode a code point. What is a code point? A codepoint is also a number composed of a single 16 bit code unit or two 16 bit code units. Each character is encoded as one or more codepoints in unicode.

A string gets an index at each code unit for holding that part of the string.

The concept of character is hard to define. For simplicity we can define it like below:

• Any symbol that doesn't seems broken.
• The invisible things that allows you to do space, tabs and newlines.

We can write a function to get the decimal values of the code units of a string:

function getCodeUnits(s) {
  let result = [];
  for (let i = 0; i < s.length; i++) {
    result.push(s.charCodeAt(i));
  }
  return result;
}

getCodeUnits("Hi 😄");
// [72, 105, 32, 55357, 56836]

getCodeUnits("I ❤️‍🔥 JS");
// [73, 32, 10084, 65039, 8205, 55357, 56613, 32, 74, 83]

Note that the 😄 emoji needs 2 code units and the ❤️‍🔥 needs 5 code units.