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Values and types

Values lie at the heart of programming. In this lesson, we'll look at some of JavaScript's primitive types of values and how to manipulate them. We'll also discuss related concepts like expressions, statements, and variables.

SlidesPDF

You can do these exercises in the Node REPL (read-eval-print loop) or in a JavaScript file.

Tip: If you’re using the REPL, you can press Ctrl + D to exit the REPL when you’re done. And if you’re using a file, don’t forget to use console.log() to view your results.

  1. Write and run the code samples in the slides to see how they work. Change things and even make intentional mistakes to see how the results change. You’ll build a good mental model of JavaScript this way. You’ll also get comfortable with writing JavaScript and recognising the errors you’ll encounter later on.

  2. Suppose we have the following string:

    const str = "Some random string";

    We’ve seen that str[0] will give us the first character, str[1] will give the second, and so on. How do we get the last character? (Hint: use the string’s length.)

  3. Given this string:

    const message = 'His name is Isa'

    Declare a new variable with the same value, but with the word ‘Isa’ replaced with ‘Amal’, and ‘His’ with ‘Her’. Of course, you could just do something like:

    const message2 = 'Her name is Amal'

    But there’s a string method to do this dynamically. Find it out and use it to replace the words.

We learnt about the const and let keywords for declaring variables, but there’s a third keyword, var, that you might find in JavaScript code:

var x = 1;

You can reassign the variable like you can with let. However, the variable behaves differently (and rather strangely) than both let and const variables.

We don’t need var for our purposes, but we may discuss it later when we look at some of the finer details of JavaScript. I’m only mentioning it here so you know it’s a thing 🙂.

When declaring a variable in some programming languages, such as C++, you must specify its type. (There are some exceptions, such as when the type can be deduced.)

int x = 1;
float y = 2.3 * 5.9;
string name = "Isa";
bool done = true;

// The right-hand side is clearly a string.
// No need to specify it, you can use auto instead.
auto surname = "Abba";

You can’t change the type of a variable after declaring it in these languages:

int x = 4;
// Error.
// x was declared as an int, you can't assign a string to it
x = "Hello";

We say that these languages are statically typed. On the other hand, languages like JavaScript that don’t have these constraints are dynamically typed.