Introduction JavaScript Keywords
JavaScript keywords are reserved words. There are numbers of reserved keywords in javascript which has some special meaning. These keywords cannot be used as identifiers for example for variable names, for loop labels or for function names, etc in our JavaScript programs.
List of Javascript keywords
The list of JavaScript keywords outlined below is:
| goto
|
In
|
instanceof
|
Static
|
finally
|
| arguments
|
public
|
Do
|
else
|
Const
|
| Function
|
class*
|
return
|
let*
|
Catch
|
| Eval
|
For
|
if
|
This
|
try
|
| Break
|
debugger
|
Yield
|
extends*
|
enum*
|
| Continue
|
export*
|
null
|
switch
|
private
|
| New
|
throw
|
while
|
Case
|
await*
|
| Delete
|
super*
|
default
|
Void
|
Var
|
| protected
|
Package
|
Interface
|
False
|
typeof
|
| implements
|
With
|
import*
|
True
|
` |
The keywords which are marked by ‘*’ are new in ECMAScript 5 and 6 (which are the version of javascript). The lists of reserved words which have been removed from the ECMAScript 5 and 6 standard are given below –
- goto
- double
- transient
- volatile
- int
- synchronized
- throws
- native
- float
- short
- byte
- Boolean
- long
- abstract
- final
- char.
Example of JavaScript Keywords
Here we will learn javascript keywords with help of variety of examples.
1. goto keyword
Used to return execution control to a specific location. In general, the goto can be accomplished by the break and continue keywords.
Example
var no=0;
sposition
document.write(" something print here ");
no++;
if(no < 10) goto sposition;Now the same code we rewrite with break and continue keywords as
var no=0;
sposition: while(true) {
document.write(" something print here ");
no++;
if(no < 10) continue sposition;
break;
}2. in keyword
It is an operator returns true if the specified property is present in the specified object, else it returns false.
Example
var fruits={f1: "apple", f2: "banana", f3: "orange"};// output as true expected
console.log('apple' in fruits);3. instanceof keyword
Returns true if the object is an instance of the class otherwise false
Example
var fruits=["apple", "banana", "orange"];
// Returns true
fruits instanceof Object;
// Returns true
fruits instanceof Array;
// Returns false
fruits instanceof String;4. arguments keyword
Represents the list of parameters passed to the function when calling the function.
Example
const func = function(p1, p2, p3) {
const param = Array.from(arguments);
console.log(param) // [11, 22, 33]
}
func(11, 22, 33);5. public keyword
It is an access modifier that can be used with attributes, classes, constructors and methods which make it accessible to other classes.
Example
public class Employee {
public String efn = "Joseph";
public String eln = "Doe";
}
class MainClass {
public static void main(String[] args) {
Employee obj = new Employee ();
System.out.println("Name= " + obj.efn + " " + obj.lname);
}
}6. Do keyword
Used to define a do-while loop.
Example
var a=1;
do {
document.write("loop is running for " + a + "times</p>");
a++;
}
while(a <= 10);7. Function keyword
Used to define a function to execute a block of code.
Example
var func = function(){
return "Hello";
}
alert(func());8. class* keyword
Used to define a class.
Example
public class Employee {
public String efn = "Joseph";
public String eln = "Doe";
}9. return keyword
Used to return from the function or method with or without a value.
Example
var func = function(){
return "Hello";
}10. Eval keyword
Used to evaluate a specified string. The eval use as a global function eval().
Example
function fun( ) {
var str1=2;
var str1=3;
var res = eval(new String(str1 + str2));
document.write(res);
}
fun();11. For keyword
Used to define a loop, for loop to repeatedly execute a block of code until a condition true.
Example
for(var a=0; a<=10; a++) {
document.write("The loop is running for " + a + " times");
}12. if keyword
Used to define a conditioned construct. if the statement is used to run a block of code if the condition is true.
Example
var date = new Date();
var day = date.getDay(); // Sunday Saturday : 0 6
if(day==5) {
alert("This is weekend!");
} else {
alert("This is non-weekend!");13. Break keyword
used into a loop to break or stop the execution of the loop.
Example
for(var a=0; a<=10; a++) {
if(a == 5)
break;
document.write("The loop is running for " + a + " times");
}14. debugger keyword
Used to stop the execution of javascript code and call debugging function if define. Debugger keyword word the same as the break.
Example
var prod = 10 * 10;
debugger;
document.getElementbyId("id").innerHTML = prod;15. Yield keyword
Used to pause and resume a generator function. The generator function is the same as a normal function but for returning a value in place of return it uses yield keyword.
Example
function* iter( a ) {
while (a < 4) {
yield a++;
}
}
const i = iter ( 1 );
console.log(i.next().value); //return 1
console.log(i.next().value); //return 2
console.log(i.next().value); //return 316. Continue keyword
Used into a loop to continue the loop and skip the following statements inside the loop.
Example
for(var a=0; a<=10; a++) {
if(a == 5)
continue;
document.write("The loop is running for " + a + " times");
}17. export* keyword
Used to export objects, functions or values from the module so that can be used in another program with the help of import statement.
Example
export let var fruits = ["apple", "banana", "orange"];// export an array
export const fruit= "apple";// export a constant18. null keyword
Used to represent a special data type no value.
Example
var age = null;
alert(age);19. New keyword
Used to create an object.
Example
Employee obj = new Employee ();20. throw keyword
Used in a try block to explicitly throw an exception object.
Example
Var i=1
try {
if(i == "") throw "is Empty";
if(x > 0) throw "positive";
if(x < 0) throw "negative";
}
catch(msg) {
message.innerHTML = "Input " + msg;
}21. while keyword
Used for while loop, while loop executes the block of code until the condition is true.
Example
var a=1;
while(a <= 10)
{
document.write("loop is running for " + a + "times</p>");
a++;
}22. Delete keyword
Used to remove properties from an object.
Example
var fruits={f1: "apple", f2: "banana", f3: "orange"};
delete fruits.f1;23. super* keyword
Used to call function or method of a parent class.
Example
super.disp(); //the disp is a method of a parent class24. default keyword
Used in a switch expression to specify the actions to be performed if no case
Example
var date = new Date();
switch(date.getDay()) {
case 6:
alert("This is weekend.");
break;
case 0:
alert("This is weekend.");
default:
alert("Looking for a weekend.");
break;
}25. protected keyword
An access modifier can be used with attributes, classes, constructors and methods which make it not accessible to other classes.
Example
public class Employee {
protected String efn = "Joseph";
}26. Package keyword
Used to identify java classes and to execute the java method in a javascript.
Example
inBlock['package'] = something;27. Interface keyword
Used to define an interface (interface contains all abstract methods).
Example
interface car
method drive(whichSpeed)
method break( )
class car1 implements car
{
// Class code here
}28. implements keyword
Used to implement the interface in a class.
Example
interface car
method drive(whichSpeed)
method break( )
class car1 implements car29. With keyword
Used for iterating, just, in short, it is shortened for iteration.
Example
var fruits = ["apple", "banana", "orange"];
for ( var i = fruits.length; i--; ) {
with ({ no : i }) {
link.onclick = function() {
alert(no);
};
}
}30. import* keyword
Used to import the module in the javascript program.
Example
import * as alias from '/modules/mymodule.js';31. typeof keyword
Which used to return the data type of an operand.
Example
typeof("hello") // output as string32. Var keyword
Used to declare a variable,
Example
var fruits = ["apple", "banana", "orange"];
var age=22;33. await* keyword
Used to wait for javascript until the promise returns its result.
Example
async function fun() {
let promise = new Promise((resolve, reject) => {
setTimeout(() => resolve(" yes, it is done!"), 100)
});
let res = await promise; // wait until the promise returns us a value
alert(result); // output give yes, it is done
}
};
fun();34. enum* keyword
Used to define a predefined list.
Example
const fruits={
APPLE: 'apple',
BANANA: 'banana',
Orange: 'orange',
}
let fruit =fruits.APPLE
if(!fruit){
throw new Error(' fruit is not defined ')
}
switch( fruit ){
case fruit.APPLE: alert("This is an apple.");
break;
case fruit.BANANA: alert("This is a banana.");
break;
case fruit.ORANGE: alert("This is an orange.");
}35. try keyword
Used for exception handling to check a block of code for errors.
Example
Var i=1
try {
if(i == "") throw "is Empty";
if(x > 0) throw "positive";
if(x < 0) throw "negative";
}
catch(msg) {
message.innerHTML = "Input " + msg;
}36. Catch keyword
Again used in exception handling to handle the error.
Example
Var i=1
try {
if(i == "") throw "is Empty";
if(x > 0) throw "positive";
if(x < 0) throw "negative";
}
catch(msg) {
message.innerHTML = " Input " + msg;
}37. finally keyword
Used in exception handling, finally block of code always execute regardless of whether the error is generating or not.
Example
Var i=1
try {
if(i == "") throw "is Empty";
if(x > 0) throw "positive";
if(x < 0) throw "negative";
}
catch(msg) {
message.innerHTML = "Input " + msg;
}
finally
{
alert("This is a finally code, which always execute.");38. Const keyword
Used to define a constant variable and that cannot be farther reassigned.
Example
const age=22;39. private keyword
IS an access modifier can be used with attributes, classes, constructors and methods which make it not accessible to other classes.
Example
public class Employee {
private String efn = "Joseph";
}
class MainClass {
public static void main(String[] args) {
Employee obj = new Employee ();
System.out.println("Name= " + obj.efn + " " + obj.lname);// gives error
}
}40. True keyword
Used to store or represent primitive data type Boolean ‘true’.
Example
var inp = true;41. False keyword
Used to store or represent primitive data type Boolean ‘false’.
Example
var inp = false;42. Void keyword
used to evaluates an expression and returns undefined. A void operator is frequently used to get the undefined primitive value.
Example
<a href = "javascript:void(0);">
click link
</a>43. Case keyword
Used in a switch-case construct, where the value of an expression compares with the case clause value and executes the statements associated with the case whose case value is matched.
Example
var date = new Date();
switch(date.getDay()) {
case 6:
alert("This is weekend.");
break;
case 0:
alert("This is weekend.");
default:
alert("Looking for a weekend.");
break;
}44. switch keyword
Used in a switch-case construct, where switch evaluates an expression.
Example
var date = new Date();
switch(date.getDay()) {
case 6:
alert("This is weekend.");
break;
case 0:
alert("This is weekend.");
default:
alert("Looking for a weekend.");
break;
}45. extends* keyword
Used in class declarations to create a class that inherits another class.
Example
class Employee extends Person {
constructor(name, eid, salary) {
super(name);
}
get incsalary() {
return this.salary * 0.2;
}
}
46. This keyword
Used to refer to the current object.
Example
class Employee extends Person {
constructor(name, eid, salary) {
super(name);
}
get incsalary() {
return this.salary * 0.2;
}
}47. let* keyword
Used to declare a variable limited to a scope of a block of code, unlike a variable declared by the var keyword.
Example
let var fruits = ["apple", "banana", "orange"];48. else keyword
Used in the if-else statement, the else indicates the block of statements to be executed if the expression evaluates false.
Example
var date = new Date();
var day = date.getDay(); // Sunday - Saturday : 0 - 6
if(day==5) {
alert("This is weekend!");
} else {
alert("This is non-weekend!");49. Static keyword
Used to define a static method in a class. Static methods are those methods that are not called on the object.
Example
class Employee extends Person { constructor(name, eid, salary) { super(name); } static disp() { return "This is static method " } } document.writeln( Employee.disp() );