JS: Statements & Expressions

Learning Goals

• Understand the difference between statements and expressions in JavaScript
• Use various operators in JavaScript

Vocabulary

• Statement A single piece of code that accomplishes one task or action
• Expression A statement that produces a value
• Operator Symbols that are used to assign, compare, and perform operations

Statements

A script is a series of instructions that a computer can follow one-by-one. Each individual instruction is known as a statement. Each statement in JavaScript is followed by a semicolon.

Example statements:

"hello";
358;
false;
console.log('BOOM');
alert('POW');

Expressions

An expression is a statement that, when read by the browser, results in a single value. Expressions can use operators to create this single value. Here are some example expressions:

This expression results in “apple”: "ap" + "ple"

This expression results in 5: 2 + 3

What is the difference?

In simpler terms, a statement performs some kind of action. It does not return anything. An expression produces a value and can be written wherever a value is expected.

Think about the following sentences:

• “It’s raining a lot!”
• “It’s raining cats and dogs!”

Both of these sentences are statements! They declare something. But one of them is an expression - it means something other than what the letters actually spell out. “It’s raining cats and dogs” ACTUALLY means “It’s raining a lot”.

Similarly, consider the following code statements:

• 4;
• 2 + 2;

They’re both statements. One is an expression - 2 + 2; evaluates to 4;!

Operators

Expressions rely on operators to calculate their single value. There are 5 basic types of operators to get you started:

1. Assignment operators assign a value to a variable. var color = 'magenta';
2. Arithmetic operators perform basic math. var addTwo = 2 + 2;
3. String operators combine strings. var greeting = 'Hello! ' + 'Nice to meet you.';
4. Comparison operators compare two values and return a true or false. var buy = 3 > 5; // Value of buy is false
5. Logical operators combines expressions and return a Boolean value of true or false. var buy = (5 > 3) && (2 < 4);