Why is “👍”.length === 2?

by

Javascript uses UTF-16 (source) to manage strings.

In UTF-16 there are 1,112,064 possible characters. Now, each character uses code points to be represented(*). In UTF-16 one code-point use two bytes (16 bits) to be saved. This means that with one code point you can have only 65536 different characters.

This means some characters has to be represented with two code points.

String.length() returns the number of code units in the string, not the number of characters.

MDN explains quite well the thing on the page about String.length()

This property returns the number of code units in the string. UTF-16, the string format used by JavaScript, uses a single 16-bit code unit to represent the most common characters, but needs to use two code units for less commonly-used characters, so it’s possible for the value returned by length to not match the actual number of characters in the string.

(*): Actually some chars, in the range 010000 – 03FFFF and 040000 – 10FFFF can use up to 4 bytes (32 bits) per code point, but this doesn’t change the answer: some chars requires more than 2 bytes to be represented, so they need more than 1 code point.

This means that some chars that need more than 16 bits have a length of 1 anyway. Like 0x03FFFF, it needs 21 bits, but it uses only one code unit in UTF-16, so its String.length is 1.

console.log(String.fromCharCode(0x03FFFF).length)

More Related Contents:

  1. Using Javascript’s atob to decode base64 doesn’t properly decode utf-8 strings
  2. How to prevent Unicode characters from rendering as emoji in HTML from JavaScript?
  3. How to detect emoji using javascript
  4. utf-8 word boundary regex in javascript
  5. Javascript: Unicode string to hex
  6. Conversion between UTF-8 ArrayBuffer and String
  7. How to remove emoji code using javascript?
  8. How can I split a string containing emoji into an array?
  9. How to convert UTF8 string to byte array?
  10. Encoding UTF-8 when exporting HTML table to Excel
  11. Printing emojis with JavaScript and HTML
  12. How to count the correct length of a string with emojis in javascript?
  13. Adding UTF-8 BOM to string/Blob
  14. Replacing accented characters with plain ascii ones [duplicate]
  15. How to convert one emoji character to Unicode codepoint number in JavaScript?
  16. How to remove invalid UTF-8 characters from a JavaScript string?
  17. Regex matching emoticons
  18. What is the difference between the `=` and `==` operators and what is `===`? (Single, double, and triple equals)
  19. Where does npm install packages?
  20. How to make exe files from a node.js app?
  21. ie8 var w= window.open() – “Message: Invalid argument.”
  22. Handling errors in express async middleware
  23. What events does an fire when its value is changed?
  24. Discord.js Bot Welcomes Member, Assign a Role and send them a DM
  25. What’s the difference between putting script in head and body?
  26. Can’t resolve module (not found) in React.js
  27. Highcharts percentage of total for simple bar chart
  28. Convert JavaScript String to be all lowercase
  29. Boolean HTML Attributes
  30. Keep tabbing within modal pane only

Leave a Comment