How to check if a string is a valid hex color representation?

For example:

AA33FF = valid hex color

Z34FF9 = invalid hex color (has Z in it)

AA33FF11 = invalid hex color (has extra characters)

/^#[0-9A-F]{6}$/i.test('#AABBCC')

To elaborate:

^ -> match beginning
# -> a hash
[0-9A-F] -> any integer from 0 to 9 and any letter from A to F
{6} -> the previous group appears exactly 6 times
$ -> match end
i -> ignore case

If you need support for 3-character HEX codes, use the following:

/^#([0-9A-F]{3}){1,2}$/i.test('#ABC')

The only difference here is that

 [0-9A-F]{6}

is replaced with

([0-9A-F]{3}){1,2}

This means that instead of matching exactly 6 characters, it will match exactly 3 characters, but only 1 or 2 times. Allowing ABC and AABBCC, but not ABCD

Combined solution :

var reg=/^#([0-9a-f]{3}){1,2}$/i;
console.log(reg.test('#ABC')); //true
console.log(reg.test('#AABBCC')); //true

// regular function
function isHexColor (hex) {
  return typeof hex === 'string'
      && hex.length === 6
      && !isNaN(Number('0x' + hex))
}

// or as arrow function (ES6+)
isHexColor = hex => typeof hex === 'string' && hex.length === 6 && !isNaN(Number('0x' + hex))

console.log(isHexColor('AABBCC'))   // true
console.log(isHexColor('AABBCC11')) // false
console.log(isHexColor('XXBBCC'))   // false
console.log(isHexColor('AAXXCC'))   // false

This answer used to throw false positives because instead of Number('0x' + hex), it used parseInt(hex, 16).
parseInt() will parse from the beginning of the string until it reaches a character that isn't included in the radix (16). This means it could parse strings like 'AAXXCC', because it starts with 'AA'.

Number(), on the other hand, will only parse if the whole string matches the radix. Now, Number() doesn't take a radix parameter, but luckily, you can prefix number literals to get a number in other radii.

Here's a table for clarification:

╭─────────────┬────────────┬────────┬───────────────────╮
│ Radix       │ Characters │ Prefix │ Will output 27    │
╞═════════════╪════════════╪════════╪═══════════════════╡
│ Binary      │ 0-1        │ 0b     │ Number('0b11011') │
│ Octal       │ 0-7        │ 0o     │ Number('0o33')    │
│ Decimal     │ 0-9        │ -      │ -                 │
│ Hexadecimal │ 0-9A-F     │ 0x     │ Number('0x1b')    │
╰─────────────┴────────────┴────────┴───────────────────╯