Concat

concat() will take N strings as parameters, combine them, and return the combined text. It works pretty much like using the + operator, but this time around if the arguments are not really Strings they will be converted directly to strings.

let string1 = 'Welcome aboard'
let string2 = 'Choo choo shoe'

string1.concat(' on the ', string2)
//output: 'Welcome aboard on the Choo choo shoe'

''.concat(1,2,3)
//output: '123'

''.concat(1,false)
//output: '1false'

Compare the code above with the next one. In the first concat() you're passing a single parameter, which will be resolved and THEN added to ''.

''.concat(1+false)
//output: '1'

''.concat(1+true)
//output: '2'

''.concat(1,true)
//output: '1true'

console.log(1 + false)
//output: 1

console.log(1 + false + '')
//output: '1'

Repeat

Can you repeat strings using concat()? Well... yeeeeah...

Take a look:

let string = 'Choo choo shoe'

''.concat(string, string, string)
//output: 'Choo choo shoeChoo choo shoeChoo choo shoe'

It works, sure. But then you have repeat().

It takes a single parameter, an integer between 0 and INFINITY which will be the number of times your string will be repeated.

let string = 'Choo choo shoe'

string.repeat(3)
//output: 'Choo choo shoeChoo choo shoeChoo choo shoe'

Much more train noise, much cleaner looking.

If you try repeat() passing 0 as a parameter, it will return an empty string. Same thing will happen if you don't feed it any parameter, or something that can't be converted into an integer.

If your repeat() count is a floating point number rather than an integer, only the integer will be considered.

let string = 'Choo choo shoe'

string.repeat(0)
//output: ''

string.repeat('2')
//output: 'Choo choo shoeChoo choo shoe'

string.repeat('Yay!')
//output: ''

string.repeat()
//output: ''

string.repeat(3.14)
//output: 'Choo choo shoeChoo choo shoeChoo choo shoe'

So it's pretty lenient, but it will throw an error if you try a negative number!

"Hi, I'm on TV!".repeat('-2')
//output: Uncaught RangeError: Invalid count value: -2

Practice

If you're up for a challenge, check this 7-kyu CodeWars kata!

For extra practice, jskatas.org has interesting exercises in the form of tests.