The const keyword
JavaScript supported only two scopes Global and Function scope prior to the introduction of let and const. ECMAScript 2015 introduced let and const. The const works very much like let but the reference to the const can not be changed.
There are few important points to keep in mind regarding the const,
- A const cannot be reassigned and a const keyword creates the read the only reference to value. If we have assigned a primitive value to const than it can not be changed.
<script>
const x=10;
document.write(x++);//ERROR
//Uncaught TypeError: Assignment to constant variable.
</script>
- A const must be assigned a value at the time of its declaration.
<script>
const x;
x=15;//ERROR
//Uncaught SyntaxError: Missing
initializer in const declaration
document.write(x);
</script>
- A const has a block Scope.
<script>
{
const a=12;
document.write(a+'<br>');
{
const a=15;
document.write(a+'<br>');
{
const a=20;
document.write(a+'<br>');
}
}
document.write(a+'<br>');
}
</script>
Output:
12
15
20
12
- The property of a const object can be changed but it cannot be changed to a reference to the new object.
<script>
// We can change the object
//before changing the value
const
student={name:'Shankar',roll:15,branch:'CSE'};
for(item in
student)
{
document.write('<br>'+student[item]);
}
student.roll=20;
//After changing the value
for(item in
student)
{
document.write('<br>'+student[item]);
}
</script>
Output:
Shankar
15
CSE
Shankar
20
CSE
- The values inside the const array can be changed, we can add new items to const arrays but it should not refer to a new array.
<script>
// We can change the content in an
array
const
colors = ["RED", "BLUE", "GREEN", "ORANGE"];
document.write(colors.toString()+'<br>');
colors[2] = "CYAN"; //
possible
document.write(colors.toString());
</script>
Output:
RED,BLUE,GREEN,ORANGE
RED,BLUE,CYAN,ORANGE
- We can redeclare a const variable inside different block scopes.
<script>
{
const v='something';
document.write(v+'<br>');
}
{
const v=51;
document.write(v+'<br>');
}
</script>
Output:
something
51
- We cannot hoist a const variable like let.
<script>
a = 30;
document.write(a);
const a;//ERROR
//Uncaught
SyntaxError: Missing initializer in const declaration
</script>
