JavaScript Understanding the Weird Parts - Built-In Function Constructors

index.js

let a = new Number(3);
a;
a.toFixed(2);
Number.prototype.toFixed(2);


let b = new String('John');
b.indexOf('o');
String.prototype.indexOf('o');

"John".length // boxing; same as new String('John').length

let c = new Date('12/31/2020');

//Add some functionality to all Strings.

String.prototype.isLengthGreaterThan = function(limit) {
  return this.length > limit; 
  // the this reference will be handled dynamically based on the object on which the function is invoked.
}


console.log(new String('Some String').isLengthGreaterThan(10));
console.log("John".isLengthGreaterThan(3));

// Many Libraries and frameworks use this feature of Javascript

Number.prototype.isPositive = function() {
  return this > 0;
}

console.log(3.isPositive()); // this fails as the boxing does not work for all types.

var num = new Number(3);
console.log(num.isPositive());

// Using built-in function constructors for primitives is a little dangerous as we get an object and not a primitive
// For dealing with Dates, use Moment.js library