Javascript New Array Functionality

Question

I came across the below code in one website and I am unable to understand the line commented as "DOUBT". How is the function returning the repeated text by using "new Array(n + 1).join(this)" ?

if (!String.prototype.repeat) { 
 String.prototype.repeat = function(n) {
 return new Array(n + 1).join(this); /* DOUBT */
 };
}
alert( "La".repeat(3) );
Output: LaLaLa

Answer 1

When you join an array of length n by an argument str, you'll create a new string by inserting n - 1 items of str, such that one is between each array item. Eg:

const arr = ['foo', 'bar', 'baz'];
arr.join(' ')
// results in
// foo bar baz

The new Array(n + 1) creates an array which has length n + 1 but has no elements. These non-existent elements, when converted into a string by .join, result in the empty string.

The this inside String.prototype.repeat is the string the function is being called on. For example:

'foo'.repeat(

results in String.prototype.repeat being called with a this of 'foo'.

So:

new Array(n + 1).join(this)

results in a new string containing n repetitions of this.

Another way of looking at it, if 'x'.repeat(2) is called, the following array is constructed:

// [<empty>, <empty>, <empty>]
// insert 'x' between each element of the array:
// '' + 'x' + '' + 'x' + ''
// result: 'xx'

• Justinas: In other words joining null values n times with separator foo

Answer 2

I am Tigran, and I can answer your question.

There is we polifying repeat method in String class:

 if (!String.prototype.repeat) { 
 String.prototype.repeat = function(n) {
 //some code
 }
}

There are this - a given string (in this example "La"):

return new Array(n + 1).join(this);

Let's discuss by "La" string, "La".repeat(3) there are this is "La" and n is 3. new Array(3 + 1) -> [empty ×ばつ 4] [,,,,].join('La') -> "LaLaLa" (, -> empty) join is method for converting array to string by given string. If there are any questions, feel free to ask.