Introduction to Types

JavaScript includes several types of values, but for now we'll focus on the three we learned about in Chapter 1: strings, numbers, and booleans. To recap:

console.log("cat"); // prints the string "cat" to the console
console.log(false); // prints the boolean value false to the console
console.log(8.76); // prints the number 8.76 to the console

Types are important because they help determine how JavaScript interprets your code. Here's a common example: javascript console.log("cat" + 5); // prints "cat5" to the console console.log("cat" * 5); // prints NaN (not a number) to the console

As you can see, JavaScript does some strange things when you apply a single operator (such as + or *) to two or more different types of operands (like "cat" and 5). This is JavaScript's attempt at type conversion, and it's a common complaint with the language. Most people think "cat" + 5 should return an error just like "cat" * 5, but it doesn't. The lesson: you need to pay attention to data types.

To check the type of a value in JavaScript, we use the operator typeof(). The typeof() operator can be used to demonstrate that true and false are boolean values:

typeof(true); // "boolean"
typeof(false); // "boolean"

We can also use typeof() to show that "dmx" is a string, and that integers (such as 8) and floats (such as 9.99) are numbers:

typeof("dmx"); // "string"
typeof(8); // "number"
typeof(9.99); // "number"

In the expressions above, typeof() is the operator; true, false, "dmx", 8, and 9.99 are all operands.

Numbers, strings, and booleans are known as the primary data types, and for good reason. Get to know them well. The other data types you'll learn about later, including objects and arrays, depend on a solid understanding of these first three.

console.log(typeof("gg"));

console.log(typeof(true));

console.log(typeof(3) === "number");

console.log(typeof(typeof(78)));