JavaScript Keywords

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 3

16. 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 constant

18. 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 class

24. 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 car

29. 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 string

32. 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()  );