Statements vs Expressions in JavaScript

Expressions

An expression evaluates into (results in) a single value.

For example, these are all expressions:

• 1 → produces 1
• "hello" → produces "hello"
• 5 * 10 → produces 50
• num > 100 → produces either true or false
• [1, 2, 3].pop() → produces the number 3

Broadly speaking there are two types of expressions -

An expression that is already a single value

When you first declare a variable using the let keyword, it is given a special value of undefined.

In order for a variable to be useful, it needs to be given a value. This is done using the assignment operator (the equals sign).

let color;
color = "red";

Here the value red is an expression.

An expression that uses two or more values to return a single value

You can perform operations on any number of individual values to determine a single value.

For example:

let area = 3 * 2;

Here the expression 3 * 2 evaluates into 6.

Another example where an expression uses two values to yield a single value would be where two strings are joined to create a single string.

Statements

A JavaScript program is a sequence of statements. Each statement is an instruction for the computer to do something.

Here are some examples of statements in JavaScript:

let hi = 5;

if (hi > 10) {
  // More statements here
}

throw new Error("Something exploded!");

Statements often have "slots" for expressions. We can put any expression we like into those slots.

For example, declaring a variable has an expression slot:

let hi = /* some expression */;

A handy trick

Want to know whether a chunk of JS is an expression or a statement? Try to log it out!

console.log(/* Some chunk of JS here */);

If it runs, the code is an expression. If you get an error, it's a statement (or, possibly, invalid JS).

In summary

A JavaScript program consists of a sequence of statements. Each statement is an instruction to do something, like create a variable, run an if/else condition, or start a loop.

Expressions produce a value, and these values are slotted into statements. Expressions are always part of a statement.

It can take a while for this distinction to become intuitive, but hopefully, this article post has clarified a few things!