JavaScript Strings cheatsheet

const a = 'single'
const b = "double"
const c = `template ${a}`    // → "template single"

const multi = `line one
line two`

String(42)     // → "42"
String(null)   // → "null"

See: Template literals, String() (developer.mozilla.org)

'hello'.length         // → 5
'hello'[1]             // → 'e'
'hello'.at(-1)         // → 'o'
'hello'.charAt(1)      // → 'e'
'hello'.charCodeAt(1)  // → 101

for (const ch of 'hi') {
  console.log(ch)    // logs "h", then "i"
}

Strings are immutable: methods return new strings, never change the original.

See: charAt(), String.prototype.at() (developer.mozilla.org)

'hello'.includes('ell')     // → true
'hello'.startsWith('he')    // → true
'hello'.endsWith('lo')      // → true
'hello'.indexOf('l')        // → 2
'hello'.lastIndexOf('l')    // → 3
'hello'.indexOf('z')        // → -1

Searches return -1 (or false) when the substring is absent.

See: includes(), startsWith(), indexOf() (developer.mozilla.org)

'hamburger'.slice(3, 6)      // → "bur"
'hamburger'.slice(-3)        // → "ger"
'hamburger'.substring(6, 3)  // → "bur"  (args swapped)
'hamburger'.substr(3, 3)     // → "bur"  (deprecated)

slice() takes negative indexes; substring() clamps them to 0.

See: slice(), substring(), substr() (developer.mozilla.org)

'Hi'.toUpperCase()        // → "HI"
'Hi'.toLowerCase()        // → "hi"
'  pad  '.trim()          // → "pad"
'  pad  '.trimStart()     // → "pad  "
'  pad  '.trimEnd()       // → "  pad"
'ab'.repeat(3)            // → "ababab"
'5'.padStart(3, '0')      // → "005"
'5'.padEnd(3, '0')        // → "500"

'e\u0301'.normalize('NFC')   // → "é" (one code point)

See: toUpperCase(), trim(), padStart() (developer.mozilla.org)

'a-b-c'.replace('-', '+')      // → "a+b-c"
'a-b-c'.replaceAll('-', '+')   // → "a+b+c"
'a-b-c'.replaceAll(/-/g, '+')  // → "a+b+c"
'm-d'.replace(/(\w+)/, '[$1]') // → "[m]-d"

replace() changes the first match only; a regex given to replaceAll() must have the g flag.

See: replace(), replaceAll() (developer.mozilla.org)

'a,b,c'.split(',')     // → ["a", "b", "c"]
'foo'.concat('bar')    // → "foobar"
['a', 'b'].join('-')   // → "a-b"

'\u{1F600}'.split('').length  // → 2  (UTF-16 units)
[...'\u{1F600}'].length       // → 1  (code points)

split('') cuts code units, so surrogate pairs break; [...str] keeps them.

See: split(), join(), concat() (developer.mozilla.org)

'a' === 'a'                // → true (compares by value)
'a' < 'b'                  // → true (code units)
'a'.localeCompare('b')     // → -1
'B'.localeCompare('a')     // → 1
'ABC'.toLowerCase()        // → "abc"

localeCompare() returns a negative, zero, or positive number for sorting.

See: localeCompare() (developer.mozilla.org)