JavaScript | Const
ES2015 (ES6) introduced const keyword to define a new variable. The difference in const variable declaration than others is that it cannot be reassigned.
Properties:
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- Cannot be reassigned.
- Block Scope
- It can be assign on the variable on declaration line.
- Primitive value.
- The property of a const object can be change but it cannot be change to reference to the new object
- The values inside the const array can be change, it can add new items to const arrays but it cannot reference to a new array.
- Re-declaring of a const variable inside different block scope is allowed.
- Cannot be Hoisted.
- Create only read only reference to value.
Example 1: It describes that the const variable cannot be reassigned.
<script type="text/javascript"> const x = 12; x = 13; x += 1;</script> |
Output:
Uncaught TypeError: Assignment to constant variable.
Example 2: It describes the const variable which contains the Block Scope.
<script type="text/javascript"> const x = 22; { const x = 90; console.log(x); { const x = 77; console.log(x); } { const x = 45; console.log(x); } } console.log(x);</script> |
Output:
90 77 45 22
Example 3: It describes the const variable and assigned it after declaration.
<script type="text/javascript"> const x; x = 12;</script> |
Output:
Uncaught SyntaxError: Missing initializer in const declaration
Example 4: It describes the const variable cannot be Hoisted.
<script type="text/javascript"> x = 3; console.log(x); const x;</script> |
Output:
Uncaught SyntaxError: Missing initializer in const declaration
Example 5: It describes that the array values can be modified only reference to array cannot be change.
<script type="text/javascript"> // Changing the content of array is // possible in cost array const arr1 = ["pankaj", "sumit", "chandan", "ajay"]; console.log(arr1.toString()); arr1[2] = "Narayan"; // possible console.log(arr1.toString());</script> |
Output:
pankaj, sumit, chandan, ajay pankaj, sumit, Narayan, ajay
Example 6: It describes that the object properties can be modified only reference to object cannot be changed.
<script type="text/javascript"> const person = { first_name: "Pankaj", last_name: "Singh", Age: 20, About: "Web Developer and Competitive Programmer" }; console.log(person); // It is possible person.first_name = "Aryan"; person.last_name = "Yadav"; person.Age = 22; person.About = "Commerce undergraduate"; console.log(person); // it is not possible // const person={ // "first_name":"Aryan", // "last_name":"Yadav", // "Age":22, // "About":"Commerce undergraduate" // }</script> |
Output:
