cheat-sheet-strings.md

Cheat sheet: strings

Strings are primitive values in JavaScript and immutable. That is, string-related operations always produce new strings and never change existing strings.

Working with strings

Literals for strings:

const str1 = 'Don\'t say "goodbye"'; // string literal
const str2 = "Don't say \"goodbye\""; // string literals
assert.equal(
  `As easy as ${123}!`, // template literal
  'As easy as 123!',
);

Backslashes are used to:

• Escape literal delimiters (first 2 lines of previous example)
• Insert special characters:
  • \\ inserts a backslash
  • \n inserts a newline
  • \r inserts a carriage return
  • \t inserts a tab

Inside a String.raw tagged template (line A), backslashes are treated as normal characters:

assert.equal(
  String.raw`\ \n\t`, // (A)
  '\\ \\n\\t',
);

Convertings values to strings:

> String(undefined)
'undefined'
> String(null)
'null'
> String(123.45)
'123.45'
> String(true)
'true'

Copying parts of a string

// There is no type for characters;
// reading characters produces strings:
const str3 = 'abc';
assert.equal(
  str3[2], 'c' // no negative indices allowed
);
assert.equal(
  str3.at(-1), 'c' // negative indices allowed
);

// Copying more than one character:
assert.equal(
  'abc'.slice(0, 2), 'ab'
);

Concatenating strings:

assert.equal(
  'I bought ' + 3 + ' apples',
  'I bought 3 apples',
);
let str = '';
str += 'I bought ';
str += 3;
str += ' apples';
assert.equal(
  str, 'I bought 3 apples',
);

JavaScript characters vs. Unicode code points

Some Unicode code points (roughly: characters) take up two JavaScript characters:

assert.equal(
  '🙂'.length, 2
);

Safe ways of processing Unicode code points:

assert.deepEqual(
  Array.from('🙂X🙂'), [ '🙂', 'X', '🙂' ]
);
assert.deepEqual(
  [...'🙂X🙂'], [ '🙂', 'X', '🙂' ]
);
for (const codePoint of '🙂X🙂') {
  console.log(codePoint);
}
// Output:
// '🙂'
// 'X'
// '🙂'

In constrast, .split('') splits into JavaScript characters:

assert.deepEqual(
  '🙂X🙂'.split(''),
  ['\uD83D', '\uDE42', 'X', '\uD83D', '\uDE42']
);

Note that the real Unicode characters are called grapheme clusters and can comprise multiple code points.

String methods

Finding substrings:

> 'abca'.includes('a')
true
> 'abca'.startsWith('ab')
true
> 'abca'.endsWith('ca')
true

> 'abca'.indexOf('a')
0
> 'abca'.lastIndexOf('a')
3

Splitting and joining:

assert.deepEqual(
  'a, b,c'.split(/, ?/),
  ['a', 'b', 'c']
);
assert.equal(
  ['a', 'b', 'c'].join(', '),
  'a, b, c'
);

Padding and trimming:

> '7'.padStart(3, '0')
'007'
> 'yes'.padEnd(6, '!')
'yes!!!'

> '\t abc\n '.trim()
'abc'
> '\t abc\n '.trimStart()
'abc\n '
> '\t abc\n '.trimEnd()
'\t abc'

Repeating and changing case:

> '*'.repeat(5)
'*****'
> '= b2b ='.toUpperCase()
'= B2B ='
> 'ΑΒΓ'.toLowerCase()
'αβγ'