javascript array join() method

Question

someone can help me to understand this script:

String.prototype.padLeft = function(width, char) {

    var result = this;
    char = char || " ";

    if (this.length < width)  {
        result = new Array(width - this.length + 1).join(char) + this;
    }
    return result;
};
console.log("abc".padLeft(10,"-"));

So.. im extend the object String with a new method. The char parameter is optional (if omitted, the function will use white space) I dont have clear this part:

result = new Array(width - this.length + 1).join(char) + this;

Am i creating a new array with 8 element that are undefined and then separate them with the separetor? Is it correct? Why there is a"+1" in the array definition? Thank you in advance

array is used mostly for the join function. It is concatenated immediately. — Ibu
– Ibu, Commented Mar 14, 2017 at 19:16
the +1 is needed because joining an array of length = n, results in n-1 separators. Ex: ['a','b','c'].join('-') results in "a-b-c". But this algorithm wants n separators, so the +1. — 001
– 001, Commented Mar 14, 2017 at 19:25

Community · Accepted Answer · 2020-06-20 09:12:55Z

1

`new Array(width - this.length + 1).join(char)`

This ^ is effectively saying "make an empty array with n number of slots, then join the empty slots together using the char to separate each empty slot. So, if char = "0", and n = 3, we get 000.

width - this.length + 1 is used to determine the number of characters needed to be added to beginning of the string.

Then we add that to the beginning of the original string: this

`+1`

You need the + 1 because of how join works.

new Array(1).join('0') = ""      // wrong
new Array(1+1).join('0') = "0"   // correct
new Array(2).join('0') = "0"     // wrong
new Array(2+1).join('0') = "00"  // correct

edited Jun 20, 2020 at 9:12

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answered Mar 14, 2017 at 19:19

Kevin Jantzer

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1 Comment

imtheman Over a year ago

The +1 is incorrect though, right? In his example it would be 10 - 3 + 1 = 8 which would make the whole string 3 + 8 = 11.

Barmar · Accepted Answer · 2017-03-14 19:40:46Z

1

When you join an array with N elements, there will be N-1 separators between the elements. The code is using join to create a string with just N separators, so you need to give it an array with N+1 elements to account for this.

answered Mar 14, 2017 at 19:40

Barmar

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javascript array join() method

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`new Array(width - this.length + 1).join(char)`

`+1`

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new Array(width - this.length + 1).join(char)

+1

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`new Array(width - this.length + 1).join(char)`

`+1`