Polyfills.md

Polyfills

1. for startsWith() method:

if (!String.prototype.startsWith) {
  String.prototype.startsWith = function (searchString, position) {
    position = position || 0;
    return this.substr(position, searchString.length) === searchString;
  };
}

• Explanation:

  • We will start by showing how the method startsWidth() work.

  "idiana".startsWith("la", 2);//true

  string.startsWith(searchString[, position]);

  searchString argument:

  The character(s) to be searched for at the start of this string or at the start point defined by position argument.

position argument:

The position in this string at which to begin searching for searchString. It is optional and defaults to 0.

-The conditional is checking if the method is on String object prototype (supported by the browser). If it is not on String object prototype (String.prototype.startsWith == undefined) ===> falsy so (!String.prototype.startsWith) will be true and excutes what code after it.

-This code will declare a startWidth method to String prototype which value is an anonymous function with 2 arguments ( same like the original startsWidth method). position will be either the value we pass in calling the method of if not passes will be equal to 0 ( same like the original method. Default is 0) position = position || 0;

• Then the function returns substr() Method.
• str.substr(start[, length]) returns the part of a string between the start index and a number of characters after it.
• So the function will return true or false depending on wether the string value from subStr(position, searchString.length) === searchString or not
• So for example in

  "diana".startsWith("la", 2);

this will return subStr(2, 2) === la start at position 2 which is character "l" to 2 characters which is "la" and this is eaqual to "la". So it returns true.