JavaScript Expressions and Statements

There are two major syntactic categories in Javascript:

1. Expressions
2. Statements

Expressions are parts of program code that can be combined and interpreted into a new value during processing. Together with statements, expression forms the building blocks of a program.

Expressions will be evaluated by the programming and returns a new value as a result of that evaluation. It is similar to arithmetical operation, in which we can do addition or subtraction between two numbers or more to create a new number as the result.

A statement performs an action. Loops and if statements are examples of statements. A program is basically a sequence of statements. Wherever JavaScript expects a statement, you can also write an expression. Such a statement is called an expression statement.

Expressions

Expressions are Javascript code snippets that result in a single value.

2 + 2 * 3 / 2

(Math.random() * (100 - 20)) + 20

functionCall()

window.history ? useHistory() : noHistoryFallback()

1+1, 2+2, 3+3

declaredVariable

true && functionCall()

true && declaredVariable

The above are expressions

Statements

Statements perform actions. In javascript, statements can never be used where a value is expected. So they cannot be used as function arguments, right-hand side of assignments, operators operand, return values…

foo(if () {return 2})

These are all javascript statements:

1. if
2. if-else
3. while
4. do-while
5. for
6. switch
7. for-in
8. with (deprecated)
9. debugger
10. variable declaration

Consider an example below and see what is an expression and what is a statement.

// first example
  let name = "Nathan" + "Sebhastian";   // this is declaration of variable name
  console.log(name);
  
  // second example
  let age = 10 + 6;   // we declare variable age here
  console.log (age);
  
  // third example
  function myFunction(a, b) {     // here we declare a function with two parameters
    if (a == "Johnny" && b == "English"){
      console.log ("Welcome, Britain's greatest agent!"); 
    }
    else {
      console.log ("Identification failed.");
    }  
  }
  myFunction ("Johnny", "English");

Time to identify expressions. Can you spot the part of our code that returns a value?

// first example
  let name = "Nathan" + "Sebhastian";   // this is declaration of variable name with string concatenation expression
  console.log(name);
  
  // second example
  let age = 10 + 6;   // we declare variable age here with integer addition expression
  console.log (age);
  
  // third example
  function myFunction(a, b) {     // here we declare a function with two parameters, this is also a function definition expressions
    if (a == "Johnny" && b == "English"){    // logical expressions                comparing the values of a and b variables
      console.log ("Welcome, Britain's greatest agent!"); 
    }
    else {
      console.log ("Identification failed.");
    }  
  }
  myFunction ("Johnny", "English"); // this function call is an expression

Now let’s look at what part of the code is a statement.

// first example
  let name = "Nathan" + "Sebhastian";
  console.log(name); // statement
  
  // second example
  let age = 10 + 6;   
  console.log (age); // statement
  
  // third example
  function myFunction(a, b) {    
  // this entire if else block is a statement
    if (a == "Johnny" && b == "English"){    
      console.log ("Welcome, Britain's greatest agent!"); 
    }
    else {
      console.log ("Identification failed.");
    }  
  }
  myFunction ("Johnny", "English");

Entire sample code are statements

// first example
  let name = "Nathan" + "Sebhastian"; // the code states that variable name's value is "Nathan" + "Sebhastian"
  console.log(name); // statement
  
  // second example
  let age = 10 + 6;   // this code states that variable age's value is 10+6
  console.log (age); // statement
  
  // third example
  function myFunction(a, b) {    
  // this entire if else block is a statement
    if (a == "Johnny" && b == "English"){    
      console.log ("Welcome, Britain's greatest agent!"); 
    }
    else {
      console.log ("Identification failed.");
    }  
  }
  myFunction ("Johnny", "English"); // statement that contain function call expression

Statements can contain an expression. That kind of statement is called an expression statement.

Resources:

Expressions versus statements in JavaScript

JavaScript Expressions and Statements

Maksim Ivanov