In order to save the DOM references to your HTML elements, you must store them in variables.

Let's breakdown the syntax to declare a variable in JavaScript.

var varName = value;
  • var: This keyword defines a variable. There are three types of variables that was introduced with the ES6 JavaScript revision. To declare a variable, developers may now define a variable as var, const, and let. We will explain in more detail below the difference between these variable types.
  • varName: Name of your variable.
  • =: Similar to most programming languages, the = symbol is used to define and set a variable name to a specific value.
  • ;: All JavaScript lines of code and statements must end with a semicolon ;.

To define an empty variable, just define the type of variable and name, and end with a semicolon (e.g var varName).

var, let, and const

In some programming languages, like Java and C++, when declaring a variable you must specify the data type (integer, String, float, boolean, etc. ) In JavaScript, a variable does not need to be specifically defined in its declaration. Instead the keyword var, const, and let define how a variable can be used and/or changed.

  • var: Variables defined by a var declaration are typically declared outside of a function and act as global variables. They can accessed by any function, redefined, and its value may change at any point in the program flow.
  • let: Variables defined by let declaration are typically defined inside of a block of code (typically defined by curly brackets,{}). This block of code is sometimes called scope. Within its scope, a let variable's value may change but cannot be redefined. This ensures that a let variable name must be unique within a single scope. let variables are typically used within functions for temporarily used and are not called in multiple functions like var variables.
  • const: Variables defined by const declaration are constant values. They cannot be updated or re-declared.

When storing DOM variables, take a moment to think about which type of variable you would use store DOM references to HTML elements.

You should use the const variable type! We don't want to be able to redefine or change the variable holding the reference to our HTML elements. This is why we should choose the const variable type.

Let's look at an example at using the querySelector method.

Example: index.html

        <title>My Document</title>

Let's say we want to store a reference to the p element.

  • Since there are no attributes attached to this element, no id or class name, this eliminates the use of the getElementById method.
  • We can only use the querySelector or querySelectorAll method. However, since there is only a single instance of a p element, we will elect to use the querySelector method with the element selector associated with its tag name p.
const pElem = document.querySelector('p');

Notice that we use either single or double quotation marks as a parameter to the querySelector method.

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