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JavaScript Variables

JS Variables

JavaScript variables are used to store data and values.

JavaScript Identifiers

All JavaScript variables must be identified with unique names. These unique names are called identifiers.

Rules for defining names for variables (unique identifiers) are:

Names can contain letters, digits, underscores, and dollar signs.
Names must begin with either a letter, an underscores or a dollar signs.
Names must not start with numbers
Names are case sensitive
Reserved words (like JavaScript keywords) cannot be used as names.
Though we can use alphabets (like x, y, Z, etc.) as variable names but is suggested that use a descriptive name for the variable (like name, price, address, etc.).

Variable Declaration

Creating a variable in JavaScript is called "declaring" a variable.

4 Ways to Declare a JavaScript Variable:

Using var
Using let
Using const
Using nothing
It is recommended that use either var, let, or const to declare a variable

					 
        
          var a;
          let b;
          const c = a + b;

          // Multiple declaration in single line
          var x, y, z;

Test Code Online

Variable Assignment

After the declaration, the variable has no value (technically it is undefined)

					 
        
          a = 5;
          b = 3;
          c = a + b;

          // Multiple assignment in single line
          x = 3, y = 2, z = x + y;

Test Code Online

Multi Line Declaration & Assignment

					 
        
          var a = 5,
          b = 5,
          c = a + b;

Test Code Online

Comparison of var, let and const Variables

All var, let and const are used to declare variables but all serve different purpose.

#	var	let	const
Assignment	Not necessary	Not necessary	Must assign value when declare
Usability	Can be used before declaration	Must declare before use	Must declare before use
Re-declare	Yes	No	No
Re-assign	Yes	Yes	No
Scope	Global	Block	Block
When To Use	Use this if you want a variable to access anywhere in the program (globally)	Use this if you do not want a variable to re-declare even outside the block scope	Use this when you know that the value must be fixed like (arrays, objects, functions, regex, etc)

Declaration & Assignment

					 
        
          var a;   // valid
          a = 5;   // valid

          let b;   // valid
          b = 5;   // valid

          const c;   // invalid
          c = 10;   // invalid

          const c = 10;   // valid

Test Code Online

Re-declaration

					 
        
          var a;   // valid
          var a = 5;   // valid

          let b;   // valid
          let b = 5;   // invalid

          const c = 5;   // valid
          const c = 5;   // invalid

Test Code Online

Re-assign

					 
        
          var a = 5;   // valid
          a = 10;   // valid

          let b = 10;   // valid
          b = 15;   // valid

          const c = 20;   // valid
          c = 30;   // invalid

Test Code Online

View JS PDF Ebooks

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JavaScript Variables

JS Variables

JavaScript Identifiers

Variable Declaration

Variable Assignment

Multi Line Declaration & Assignment

Comparison of var, let and const Variables

Declaration & Assignment

Re-declaration

Re-assign

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