为什么 JavaScript 的字符串使用 UTF-16 而一个字符的实际大小可能只有一个字节？

Question

according to this article :

Internally, JavaScript source code is treated as a sequence of UTF-16 code units.

And this IBM doc says that:

UTF-16 is based on 16-bit code units. Therefore, each character can be 16 bits (2 bytes) or 32 bits (4 bytes).

But I tested in Chrome's console that English letters are only taking 1 byte, not 2 or 4.

new Blob(['a']).size === 1

I wonder why that is the case? Am I missing something here?

Answer 1

Internally, JavaScript source code is treated as a sequence of UTF-16 code units.

Note that this is referring to source code, not String values. String values are referenced to also be UTF-16 later in the article:

When a String contains actual textual data, each element is considered to be a single UTF-16 code unit.

The discrepancy here is actually in the Blob constructor. From MDN :

Note that strings here are encoded as UTF-8, unlike the usual JavaScript UTF-16 strings.

Answer 2

UTF has a varying character size.

a has a size of 1 byte, but ą for example has 2

 console.log('a', new Blob(['a']).size) console.log('ą', new Blob(['ą']).size)