Expression and statement in JavaScript

Expression

An expression is a syntactic entity or unit of code that may be evaluated in order to determine its value. It is a combination of one or more constants, variables, functions, and operators (according to its particular rules of precedence and association).

examples:

var x = 7

x = 7is an expression that uses the = operator to assign the value seven to the variable x. The expression itself evaluates to seven.

var x = a + b * 10

x = a + b * 10is an expression that uses the = operator to assign the value
a + b * 10to the variable x. In expression value , a b 10 are operands and + - are operators.

The code 3 + 4 is also an example of expression type. This expression uses the + operator to add three and four together without assigning the result, seven, to a variable.

Types of Expression

Arithmetic Expressions:
Arithmetic expressions evaluates to a number (Generally uses arithmetic operators.). Examples

1;     
// Here 10 is an expression that is evaluated as number by the JS interpreter
1+2; 
// This is another expression that is evaluated to number 3

String Expression:
Evaluates to a character string. Examples

"Names"
"123"
"Java" + "Script" // evaluates to the string 'JavaScript'

Logical Expression:
Evaluates to true or false.

Logical AND (&&)
Logical OR (||)
Logical NOT (!)
1<2
x==10

Primary expressions:
Basic keywords and general expressions in JavaScript ,these are constant or literal values, certain language keywords, and variable references.

1.23         // A number literal
"hello"      // A string literal
/pattern/    // A regular expression literal
true      // Evalutes to the boolean true value
false     // Evaluates to the boolean false value
null      // Evaluates to the null value
this      // Evaluates to the "current" object
i             // Evaluates to the value of the variable i.
sum           // Evaluates to the value of the variable sum.
undefined     // undefined is a global variable, not a keyword like null.

Left-hand-side expressions:
Left values are the destination of an assignment

Statement

A statement performs an action(an instruction to tell the browser to what action to perform.). 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. The reverse does not hold: you cannot write a statement where JavaScript expects an expression. For example, an if statement cannot become the argument of a function.

JavaScript statements are composed of: Values, Operators, Expressions, Keywords, and Comments.

document.getElementById(“demo”).innerHTML = “JavaScript.”;
//This statement tells the browser to write “JavaScript.” inside an HTML element with id=”demo”:

Conditional statement vs Conditional Expression:

if-else statements.

var salutation;
if (male) {
    salutation = 'Mr.';
} else {
    salutation = 'Mrs.';
}

Above Conditional statements example are equivalent to the following code that is Conditional Expression:

var salutation = (male ? 'Mr.' : 'Mrs.');

The code between the equals sign and the semicolon is an expression.

Function Declarations vs Function Expression:

Function Declaration : A function, declared as a separate statement, in the main code flow.

function sayHi() {
  alert( "Hello" );
}

Function Expression: A function, created inside an expression or inside another syntax construct. It allows us to create a new function in the middle of any expression. A Function Declaration can be called earlier than it is defined. A Function Expression is created when the execution reaches it and is usable only from that moment.

For example: Here, the function is created at the right side of the “assignment expression” =.

let sayHi = function() {
  alert( "Hello" );
};

As the function creation happens in the context of the assignment expression (to the right side of =), this is a Function Expression.