String API: string.startsWith()

str.startsWith(searchString) determines whether str begins with searchString.

the 1st parameter, the string to search for

can be just a character

const s = 'the string s'; const actual = s.starts_with('t'); assert.equal(actual, true);

can be a string

const s = 'the string s'; const actual = '???'; assert.equal(actual, s.startsWith('the'));

can contain unicode characters

const nuclear = 'NO ☢ NO'; assert.equal(nuclear.startsWith('☢'), true);

a regular expression throws a TypeError

const aRegExp = 'the'; assert.throws(() => {''.startsWith(aRegExp)}, TypeError);

the 2nd parameter, the position where to start searching from

e.g. find "str" at position 4

const s = 'the string s'; const position = 3; assert.equal(s.startsWith('str', position), true);

for undefined is the same as 0

const s = 'the string s'; const myUndefined = '1'; assert.equal(s.startsWith('the', myUndefined), true);

the parameter gets converted to an int

const s = 'the string s'; const position = 'four'; assert.equal(s.startsWith('str', position), true);

a value larger than the string`s length, returns false

const s = 'the string s'; const actual = true; assert.equal(actual, s.startsWith(' ', s.length + 1));

this functionality can be used on non-strings too

e.g. a boolean

let aBool; assert.equal(String.prototype.startsWith.call(aBool, 'false'), true);

e.g. a number

let aNumber = 19; assert.equal(String.prototype.startsWith.call(aNumber + 84, '1984'), true);

also using the position works

const position = 0; assert.equal(String.prototype.startsWith.call(1994, '99', position), true);

Links

spec The official specification, actually quite good to read for this function.
mdn docs The Mozilla Developer Network docs, contains good examples.

Required Knowledge

• basics (Arrow functions)
• `let` declaration (Block scope)
• `const` declaration (Block scope)
• as parameter (Rest operator)
• with arrays (Spread operator)

Related Katas

Template strings

• basics
• multiline
• tagged template strings
• raw property

String API

• string.includes()
• string.repeat(count)
• string.startsWith()
• string.endsWith()
• String.raw
• string.trimStart()
• string.matchAll()

Difficulty Level

BEGINNER

First Published

26 August 2015

Stats

11 tests to solve