Using JS Objects and Methods

  • In lesson 1.3 discussing data types, we learned how to create objects in JavaScript.
  • Remember that objects store multiple values within one variable, and their values are attributed to their “properties”
  • We can obtain the value of an object’s property by typing the objectName.objectProperty
  • There is a difference between typical variables and objects that we have not yet discussed, and that is the difference between primitive values and reference values. —
  • Primitive values are simply values stored to variables without creating new objects.
  • For all of the types of variables we have created so far, there was a way for us to declare those variables as objects.
  • If we declare variables as objects, they would be of the reference type instead of primitive.
  • For example, lets simply declare a new variable that stores a string of text:
let primitiveWords = "Have a really great day, like maybe even the best day ever, because why not.";
  • The variable ‘primitiveWords’ is a string primitive. —
  • JavaScript identifies six data types as primitive: undefined, null, boolean, number, string, and symbol. That means the following variables are also primitive types: let year = 2018; let snowyOutside = false; let randomVariable; let placeHolder = null; —
  • Now lets create the reference type equivalent of the primitive variable ‘words’: let referenceWords = new String("Have a really great day, like maybe even the best day ever, because why not.");
  • the variable referenceWords is a String Object.
  • The purpose of learning about the difference between primitive and reference types is to understand the use of JavaScript methods.
  • JavaScript methods exist for the different objects in the JavaScript library.
  • An object’s method is essentially a function that was programmed as a property of that object, and therefore we can invoke the function similarly to how we retrieve any property of an object: objectName.objectMethod()
  • We have not yet covered functions, so we will not learn about writing object methods until the next section.
  • Instead, this lesson focuses on using the object methods already built into the JavaScript library. —
  • Let’s start with the String Object and its methods:
  • As you saw, we can create a reference type variable that stores a string object.
  • On this object, we can use the methods created for JavaScript String Objects, such as:

.length : returns the length of a string

.indexOf(“word”) : returns the index of the first occurrence of the specified text within quotes

.slice(start, end) : splits a string into two strings, the new string containing the text within the start and end indexes

  • For example: let referenceWords = new String("Have a really great day, like maybe even the best day ever, because why not."); console.log(referenceWords.length); // this prints 76 to the console console.log(referenceWords.indexOf("great")); // this prints 14 to the console console.log(referenceWords.indexOf("why")); // this prints 68 to the console
  • However, if I try to use these methods with the primitiveWords variable…. let primitiveWords = "Have a really great day, like maybe even the best day ever, because why not."; console.log(primitiveWords.length); // this prints 76 to the console console.log(primitiveWords.indexOf("great")); // this prints 14 to the console console.log(primitiveWords.indexOf("why")); // this prints 68 to the console
  • The methods still work even though primitiveWords is a primitive type!!!
  • This is because, whenever these methods are invoked, JavaScript treats primitive values as objects in order to use the many methods in the JS library.
  • This means that we can apply the JavaScript methods for most objects to their primitive equivalent.
  • The rest of this module will cover pre-existing methods that can be used on different types of objects. —

Exercise 3.3

See the Pen Exercise 3.3 by LSU DDEM (@lsuddem) on CodePen.