Statements

Learning Objectives

• Understand what is meant by the programming term statement
• Understand what is meant by "JavaScript evaluates a statement"
• Understand what is meant by "a statement has side-effects"

Statements

In programming, a statement is an individual task or instruction for the computer to complete.

A JavaScript statement is usually a single line of code, finished with a semi-colon (;).

When your JavaScript code gets executed, the browser will read the code line by line, statement by statement, instruction by instruction, until all of the instructions in the program have been executed.

Here are some examples of simple JavaScript statements:

• 5;
• 10 + 4;
• console.log('Hello World');

Evaluation

Every statement in JavaScript has a value. When we execute a statement, JavaScript will figure out what the value of that statement is. This process is called evaluation.

For example, the statement 1 + 2 evaluates to the value 3.

Very soon, we will learn how we can use the values JavaScript calculates for us to control how our programs behave.

In your developer console, when you execute a statement, the value of that statement is displayed on the line below the statement, next to a little arrow pointing left.

In some cases, the evaluation value of a statement will be more obvious than in other cases.

Let's have a go evaluating the three statements above:

In the simplest example, the statement 5 evaluates to the value 5.

In this example, the statement 10 + 4 evaluates to the value 14. This is a little more complicated, but is not really a surprise.

In the final example, the statement results in two things being printed out - Hello World, and undefined. The evaluation value of the statement is actually undefined. We can see this in the console because undefined has the little left-facing arrow beside it.

undefined

When we see the value undefined in JavaScript, this means there is no data. It's a value that has no value!

Side Effects

We have seen that we can use console.log to print messages, but this printing process is actually separate to the evaluation of the statement.

When a statement causes something to happen other than a simple evaluation, the extra thing that happens is called a side-effect.

We can say that the statement console.log('Hello World!'); was evaluated as undefined, but it has the side-effect of causing a message to printed to the screen.

Side effects are very important in programming, as they allow us to interact with the person using the program!

It's very important to understand this distinction, particularly when using console.log.