Get names of all keys in the collection and subdocuments

I need to get all the field names from my collection, including fields in subdocuments. I am currently running this command and getting only the names of the root fields. How can I get all field names?

I need to get all the field names from my collection, including fields in subdocuments. I am currently running this command and getting only the names of the root fields. How can I get all field names?

My collection looks like this:

"UserId" : 11111,
"Personal" : {
    "Email" : "[email protected]",
    "FirstName" : "Name",
    "LastName" : "Last"
},
"Car" : {
    "CarType" : "NULL"
}

This is the command

var mr = db.runCommand({
"mapreduce" : "myCollection",
  "map" : function() {
    for (var key in this) { emit(key, null); }
  },
  "reduce" : function(key, stuff) { return null; }, 
  "out": "myCollection" + "_keys"
})

db[mr.result].distinct("_id")

I want this result:

UserId, Personal.Email, Personal.FirstName, Personal.LastName, Car.CarType

MongoDB JavaScript tutorial with Examples

MongoDB JavaScript tutorial with Examples

MongoDB JavaScript tutorial shows how to do create programs that work with MongoDB in JavaScript. This tutorial uses the native mongodb driver. (There are also other solutions such as Mongoose or Monk.)

MongoDB

MongoDB is a NoSQL cross-platform document-oriented database. It is one of the most popular databases available. MongoDB is developed by MongoDB Inc. and is published as free and open-source software.

A record in MongoDB is a document, which is a data structure composed of field and value pairs. MongoDB documents are similar to JSON objects. The values of fields may include other documents, arrays, and arrays of documents. MongoDB stores documents in collections. Collectionsare analogous to tables in relational databases and documents to rows.

Installing MongoDB server

The following command can be used to install MongoDB on a Debian-based Linux.

$ sudo apt-get install mongodb

The command installs the necessary packages that come with MongoDB.

$ sudo service mongodb status
mongodb start/running, process 975

With the sudo service mongodb status command we check the status of the mongodb server.

$ sudo service mongodb start
mongodb start/running, process 6448

The mongodb server is started with the sudo service mongodb start command.

MongoDB driver install

We set up the project.

$ node -v
v11.5.0

We use Node.js version 11.5.0.

$ npm i mongodb

We install the mongodb native JavaScript driver. The npm is a Node.js package manager. The MongoDB Node.js driver provides both callback based as well as Promised based interaction.

MongoDB create database

The mongo tool is an interactive JavaScript shell interface to MongoDB, which provides an interface for systems administrators as well as a way for developers to test queries and operations directly with the database.

$ mongo testdb
MongoDB shell version v4.0.7
...
> db
testdb
> db.cars.insert({name: "Audi", price: 52642})
> db.cars.insert({name: "Mercedes", price: 57127})
> db.cars.insert({name: "Skoda", price: 9000})
> db.cars.insert({name: "Volvo", price: 29000})
> db.cars.insert({name: "Bentley", price: 350000})
> db.cars.insert({name: "Citroen", price: 21000})
> db.cars.insert({name: "Hummer", price: 41400})
> db.cars.insert({name: "Volkswagen", price: 21600})

We create a testdb database and insert eight documents in the cars collection.

MongoDB promise

Promise is an object used for deferred and asynchronous computations. It represents an operation that has not completed yet, but is expected in the future.

asyncFunc()
  .then(value => { /* success */ })
  .catch(error => { /* failure */ })
  .finally( => { /* cleanup */};

The then() method always returns a Promise, which enables us to chain method calls.

Note: the MongoClient's connect returns a promise if no callback is passed.

We can also use async/await syntax to work with promises.

MongoDB JS driver

In the first example, we print the version of the Node.js driver.

driver_version.js

const mongo = require('mongodb');
const MongoClient = mongo.MongoClient;

const url = 'mongodb://localhost:27017';

MongoClient.connect(url, { useNewUrlParser: true }, (err, client) => {

if (err) throw err;

console.log(client.topology.clientInfo);

client.close();

});

In the example, we connect to the server and find out the client information.

const mongo = require('mongodb');

We use the mongodb module.

const client = mongo.MongoClient;

MongoClient is used to connect to the MongoDB server.

const url = 'mongodb://localhost:27017';

This is the URL to the database. The 27017 is the default port on which the MongoDB server listens.

MongoClient.connect(url, { useNewUrlParser: true }, (err, client) => {

A connection to the database is created with connect().

$ node driver_version.js
{ driver: { name: 'nodejs', version: '3.2.2' },
os:
{ type: 'Windows_NT',
name: 'win32',
architecture: 'x64',
version: '10.0.17134' },
platform: 'Node.js v11.5.0, LE' }

The driver version is 3.2.2.

MongoDB list database collections

The listCollections() method lists available collections in a database.

list_collections.js

const mongo = require('mongodb');

const MongoClient = mongo.MongoClient;

const url = 'mongodb://localhost:27017';

MongoClient.connect(url, { useNewUrlParser: true }, (err, client) => {

if (err) throw err;

const db = client.db("testdb");

db.listCollections().toArray().then((docs) => {

    console.log('Available collections:');
    docs.forEach((doc, idx, array) => { console.log(doc.name) });

}).catch((err) => {

    console.log(err);
}).finally(() => {

    client.close();
});

});

The example connects to the testdb database and retrieves all its collections.

db.listCollections().toArray().then((docs) => {

console.log('Available collections:');
docs.forEach((doc, idx, array) => { console.log(doc.name) });

...

The listCollection() method finds all the collections in the testdb database; they are printed to the console.

Note: that we should be careful about using toArray() method because it can cause a lot of memory usage.

}).catch((err) => {

console.log(err);

}).finally(() => {

client.close();

});

In the catch block we catch any potential exceptions and we close the connection to the database in the finally block.

Note: our applications are console programs; therefore, we close the connection at the end of the program. In web applications, the connections should be reused.

$ node list_collections.js
Available collections:
continents
cars
cities

In our database, we have these three collections.

MongoDB database statistics

The dbstats() method gets statistics of a database.

dbstats.js

const mongo = require('mongodb');

const MongoClient = mongo.MongoClient;
const url = 'mongodb://localhost:27017';

MongoClient.connect(url, { useNewUrlParser: true }, (err, client) => {

if (err) throw err;

const db = client.db("testdb");

db.stats((err, stats) => {

    if (err) throw err;

    console.log(stats);

    client.close();
})

});

The example connects to the testdb database and shows its statistics.

$ node dbstats.js
{ db: 'testdb',
collections: 3,
views: 0,
objects: 18,
avgObjSize: 57.888888888888886,
dataSize: 1042,
storageSize: 69632,
numExtents: 0,
indexes: 3,
indexSize: 69632,
fsUsedSize: 136856346624,
fsTotalSize: 254721126400,
ok: 1 }

This is a sample output.

MongoDB find

The find() function creates a cursor for a query that can be used to iterate over results from MongoDB.

find_all.js

const mongo = require('mongodb');

const MongoClient = mongo.MongoClient;

const url = 'mongodb://localhost:27017';

MongoClient.connect(url, { useNewUrlParser: true }, (err, client) => {

if (err) throw err;

const db = client.db("testdb");

db.collection('cars').find({}).toArray().then((docs) => {

    console.log(docs);

}).catch((err) => {

    console.log(err);
}).finally(() => {

    client.close();
});

});

In the example, we retrieve all docs from cars collection.

db.collection('cars').find({}).toArray().then((docs) => {

Passing an empty query returns all documents.

$ node find_all.js
[ { _id: 5cfcfc3438f62aaa09b52175, name: 'Audi', price: 52642 },
{ _id: 5cfcfc3a38f62aaa09b52176, name: 'Mercedes', price: 57127 },
{ _id: 5cfcfc3f38f62aaa09b52177, name: 'Skoda', price: 9000 },
{ _id: 5cfcfc4338f62aaa09b52178, name: 'Volvo', price: 29000 },
{ _id: 5cfcfc4838f62aaa09b52179, name: 'Bentley', price: 350000 },
{ _id: 5cfcfc4b38f62aaa09b5217a, name: 'Citroen', price: 21000 },
{ _id: 5cfcfc4f38f62aaa09b5217b, name: 'Hummer', price: 41400 },
{ _id: 5cfcfc5438f62aaa09b5217c,
name: 'Volkswagen',
price: 21600 } ]

This is the output.

MongoDB count documents

The count() function returns the number of matching documents in the collection.

count_documents.js

const mongo = require('mongodb');
const MongoClient = mongo.MongoClient;

const url = 'mongodb://localhost:27017';

MongoClient.connect(url, { useNewUrlParser: true }, (err, client) => {

if (err) throw err;

const db = client.db("testdb");

db.collection('cars').find({}).count().then((n) => {

    console.log(`There are ${n} documents`);

}).catch((err) => {

    console.log(err);
}).finally(() => {

    client.close();
});

});

The example counts the number of documents in the cars collection.

$ node count_documents.js
There are 8 documents

There are eight documents in the cars collection now.

MongoDB findOne

The findOne() method returns one document that satisfies the specified query criteria. If multiple documents satisfy the query, this method returns the first document according to the natural order which reflects the order of documents on the disk.

find_one.js

const MongoClient = require('mongodb').MongoClient;

const url = 'mongodb://localhost:27017';

MongoClient.connect(url, { useNewUrlParser: true }, (err, client) => {

if (err) throw err;

const db = client.db("testdb");

let collection = db.collection('cars');
let query = { name: 'Volkswagen' }

collection.findOne(query).then(doc => {

    console.log(doc);

}).catch((err) => {

    console.log(err);
}).finally(() => {

    client.close();
});

});

The example reads one document from the cars collection.

let query = { name: 'Volkswagen' }

The query contains the name of the car—Volkswagen.

collection.findOne(query).then(doc => {

The query is passed to the findOne() method.

$ node find_one.js
{ _id: 8, name: 'Volkswagen', price: 21600 }

This is the output of the example.

MongoDB async/await example

With async/await we can easily work with promises in a synchronous manner.

async_await.js

const MongoClient = require('mongodb').MongoClient;

const url = 'mongodb://localhost:27017';

async function findCar() {

const client = await MongoClient.connect(url, { useNewUrlParser: true })
    .catch(err => { console.log(err); });

if (!client) {
    return;
}

try {

    const db = client.db("testdb");

    let collection = db.collection('cars');

    let query = { name: 'Volkswagen' }

    let res = await collection.findOne(query);

    console.log(res);

} catch (err) {

    console.log(err);
} finally {

    client.close();
}

}

findCar();

The example reads one document using async/await.

async function findCar() {

The function has the async keyword.

let res = await collection.findOne(query);

With await, we wait for the result of the findOne() function.

MongoDB query operators

It is possible to filter data using MongoDB query operators such as $gt$lt, or $ne.

read_gt.js

const mongo = require('mongodb');

const MongoClient = mongo.MongoClient;

const url = 'mongodb://localhost:27017';

MongoClient.connect(url, { useNewUrlParser: true }, (err, client) => {

if (err) throw err;

const db = client.db("testdb");

let query = { price: { $gt: 30000 } };

db.collection('cars').find(query).toArray().then((docs) => {

    console.log(docs);

}).catch((err) => {

    console.log(err);
}).finally(() => {

    client.close();
});

});

The example prints all documents whose car prices' are greater than 30,000.

let query = { price: { $gts: 30000 } };

The $gt operator is used to get cars whose prices are greater than 30,000.

$ node read_gt.js
[ { _id: 5d03e40536943362cffc84a7, name: 'Audi', price: 52642 },
{ _id: 5d03e40a36943362cffc84a8, name: 'Mercedes', price: 57127 },
{ _id: 5d03e41936943362cffc84ab, name: 'Bentley', price: 350000 },
{ _id: 5d03e42236943362cffc84ad, name: 'Hummer', price: 41400 } ]

This is the output of the example. Only cars more expensive than 30,000 are included.

The $and logical operator can be used to combine multiple expressions.

read_gt_lt.js

const mongo = require('mongodb');
const MongoClient = mongo.MongoClient;

const url = 'mongodb://localhost:27017';

MongoClient.connect(url, { useNewUrlParser: true }, (err, client) => {

if (err) throw err;

const db = client.db("testdb");

let query = { $and: [{ price: { $gt: 20000 } }, { price: { $lt: 50000 } }] };

db.collection('cars').find(query).toArray().then((docs) => {

  console.log(docs);

}).catch((err) => {

  console.log(err);

}).finally(() => {

  client.close();

});
});

In the example, we retrieve cars whose prices fall between 20,000 and 50,000.

let query = { $and: [{ price: { $gt: 20000 } }, { price: { $lt: 50000 } }] };

The $and operator combines $gt and $lt to get the results.

$ node read_gt_lt.js
[ { _id: 5d03e41336943362cffc84aa, name: 'Volvo', price: 29000 },
{ _id: 5d03e41e36943362cffc84ac, name: 'Citroen', price: 21000 },
{ _id: 5d03e42236943362cffc84ad, name: 'Hummer', price: 41400 },
{ _id: 5d03e42636943362cffc84ae,
name: 'Volkswagen',
price: 21600 } ]

This is the output of the example.

MongoDB projections

Projections determine which fields are passed from the database.

projections.js

const mongo = require('mongodb');
const MongoClient = mongo.MongoClient;

const url = 'mongodb://localhost:27017';

MongoClient.connect(url, { useNewUrlParser: true }, (err, client) => {

if (err) throw err;

const db = client.db("testdb");

db.collection('cars').find({}).project({_id: 0}).toArray().then((docs) => {

  console.log(docs);

}).catch((err) => {

  console.log(err);

}).finally(() => {

  client.close();

});
});

The example excludes the _id field from the output.

db.collection('cars').find({}).project({_id: 0}).toArray().then((docs) => {

The project() method sets a projection for the query; it excludes the _id field.

$ node projections.js
[ { name: 'Audi', price: 52642 },
{ name: 'Mercedes', price: 57127 },
{ name: 'Skoda', price: 9000 },
{ name: 'Volvo', price: 29000 },
{ name: 'Bentley', price: 350000 },
{ name: 'Citroen', price: 21000 },
{ name: 'Hummer', price: 41400 },
{ name: 'Volkswagen', price: 21600 } ]

This is the output for the example.

MongoDB limit data output

The limit() method specifies the number of documents to be returned and the skip() method the number of documents to skip.

skip_limit.js

const mongo = require('mongodb');
const MongoClient = mongo.MongoClient;

const url = 'mongodb://localhost:27017';

MongoClient.connect(url, { useNewUrlParser: true }, (err, client) => {

if (err) throw err;

const db = client.db("testdb");

db.collection('cars').find({}).skip(2).limit(5).toArray().then((docs) => {

  console.log(docs);

}).catch((err) => {

  console.log(err);

}).finally(() => {

  client.close();

});
});

The example reads from the cars collection, skips the first two documents, and limits the output to five documents.

db.collection('cars').find({}).skip(2).limit(5).toArray().then((docs) => {

The skip() method skips the first two documents and the limit() method limits the output to five documents.

$ node skip_limit.js
[ { _id: 5d03e40f36943362cffc84a9, name: 'Skoda', price: 9000 },
{ _id: 5d03e41336943362cffc84aa, name: 'Volvo', price: 29000 },
{ _id: 5d03e41936943362cffc84ab, name: 'Bentley', price: 350000 },
{ _id: 5d03e41e36943362cffc84ac, name: 'Citroen', price: 21000 },
{ _id: 5d03e42236943362cffc84ad, name: 'Hummer', price: 41400 } ]

This is the output of the example.

MongoDB aggregations

Aggregations calculate aggregate values for the data in a collection.

sum_all_cars.js

const mongo = require('mongodb');
const MongoClient = mongo.MongoClient;

const url = 'mongodb://localhost:27017';

MongoClient.connect(url, { useNewUrlParser: true }, (err, client) => {

if (err) throw err;

const db = client.db("testdb");

let myagr = [{$group: {_id: 1, all: { $sum: "$price" } }}];

db.collection('cars').aggregate(myagr).toArray().then((sum) => {

  console.log(sum);

}).catch((err) => {

  console.log(err);

}).finally(() => {

  client.close();

});
});

The example calculates the prices of all cars in the collection.

let myagr = [{$group: {_id: 1, all: { $sum: "$price" } }}];

The $sum operator calculates and returns the sum of numeric values. The $group operator groups input documents by a specified identifier expression and applies the accumulator expression(s), if specified, to each group.

db.collection('cars').aggregate(myagr).toArray().then((sum) => {

The aggregate() function applies the aggregation operation on the cars collection.

$ node sum_all_cars.js
[ { _id: 1, all: 581769 } ]

The sum of all prices is 581,769.

We can use the $match operator to select specific cars to aggregate.

sum_two_cars.js

const mongo = require('mongodb');
const MongoClient = mongo.MongoClient;

const url = 'mongodb://localhost:27017';

MongoClient.connect(url, { useNewUrlParser: true }, (err, client) => {

if (err) throw err;

const db = client.db("testdb");

let myagr = [
    { $match: { $or: [{ name: "Audi" }, { name: "Volvo" }] } },
    { $group: { _id: 1, sum2cars: { $sum: "$price" } } }
];

db.collection('cars').aggregate(myagr).toArray().then((sum) => {

    console.log(sum);
}).catch((err) => {

    console.log(err);
}).finally(() => {

    client.close();
});

});

The example calculates the sum of prices of Audi and Volvo cars.

let myagr = [
{ $match: { $or: [{ name: "Audi" }, { name: "Volvo" }] } },
{ $group: { _id: 1, sum2cars: { $sum: "$price" } } }
];

The expression uses $match$or$group, and $sum operators to do the task.

$ node sum_two_cars.js
[ { _id: 1, sum2cars: 81642 } ]

The sum of the two cars' prices is 81,642.

MongoDB insertOne

The insertOne() method inserts a single document into a collection.

insert_one.js

const mongo = require('mongodb');
const MongoClient = mongo.MongoClient;
const ObjectID = mongo.ObjectID;

const url = 'mongodb://localhost:27017';

MongoClient.connect(url, { useNewUrlParser: true }, (err, client) => {

if (err) throw err;

const db = client.db("testdb");

let doc = {_id: new ObjectID(), name: "Toyota", price: 37600 };

db.collection('cars').insertOne(doc).then((doc) => {

    console.log('Car inserted')
    console.log(doc);
}).catch((err) => {

    console.log(err);
}).finally(() => {

    client.close();
});

});

The example inserts one car into the cars collection.

let doc = {_id: new ObjectID(), name: "Toyota", price: 37600 };

This is a document to be inserted. A new Id is generated with ObjectID.

db.collection('cars').insertOne(doc).then((doc) => {

The insertOne() function inserts the document into the collection.

> db.cars.find({name:'Toyota'})
{ "_id" : ObjectId("5d03d4321f9c262a50e671ee"), "name" : "Toyota", "price" : 37600 }

We confirm the insertion with the mongo tool.

MongoDB insertMany

The insertMany() functions inserts multiple documents into a collection.

insert_many.js

const mongo = require('mongodb');
const MongoClient = mongo.MongoClient;
const ObjectID = mongo.ObjectID;

const url = 'mongodb://localhost:27017';

MongoClient.connect(url, { useNewUrlParser: true }, (err, client) => {

if (err) throw err;

const db = client.db("testdb");

let collection = db.collection('continents');

let continents = [
    { _id: new ObjectID(), name: "Africa" }, { _id: new ObjectID(), name: "America" },
    { _id: new ObjectID(), name: "Europe" }, { _id: new ObjectID(), name: "Asia" },
    { _id: new ObjectID(), name: "Australia" }, { _id: new ObjectID(), name: "Antarctica" }
];

collection.insertMany(continents).then(result => {

    console.log("documents inserted into the collection");
}).catch((err) => {

    console.log(err);
}).finally(() => {

    client.close();
});

});

The example creates a continents collection and inserts six documents into it.

let collection = db.collection('continents');

The collection() method retrieves a collection; if the collection does not exist, it is created.

let continents = [
{ _id: new ObjectID(), name: "Africa" }, { _id: new ObjectID(), name: "America" },
{ _id: new ObjectID(), name: "Europe" }, { _id: new ObjectID(), name: "Asia" },
{ _id: new ObjectID(), name: "Australia" }, { _id: new ObjectID(), name: "Antarctica" }
];

This is an array of six records to be inserted into the new collection. The ObjectID() creates a new ObjectID, which is a unique value used to identify documents instead of integers.

collection.insertMany(continents).then(result => {

console.log("documents inserted into the collection");

}).catch((err) => {

console.log(err);

}).finally(() => {

client.close();

});

The insertMany() method inserts the array of documents into the continents collection.

> db.continents.find()
{ "_id" : ObjectId("5cfcf97732fc4913748c9669"), "name" : "Africa" }
{ "_id" : ObjectId("5cfcf97732fc4913748c966a"), "name" : "America" }
{ "_id" : ObjectId("5cfcf97732fc4913748c966b"), "name" : "Europe" }
{ "_id" : ObjectId("5cfcf97732fc4913748c966c"), "name" : "Asia" }
{ "_id" : ObjectId("5cfcf97732fc4913748c966d"), "name" : "Australia" }
{ "_id" : ObjectId("5cfcf97732fc4913748c966e"), "name" : "Antarctica" }

The continents collection has been successfully created.

MongoDB deleteOne

The deleteOne() method is used to delete a document.

delete_one.js

const mongo = require('mongodb');
const MongoClient = mongo.MongoClient;

const url = 'mongodb://localhost:27017';

MongoClient.connect(url, { useNewUrlParser: true }, (err, client) => {

if (err) throw err;

const db = client.db("testdb");

let query = { name: "Volkswagen" };

db.collection('cars').deleteOne(query).then((result) => {

    console.log('Car deleted');
    console.log(result);
}).catch((err) => {

    console.log(err);
}).finally(() => {

    client.close();
});

});

The example deletes a document.

let query = { name: "Volkswagen" };

db.collection('cars').deleteOne(query).then((result) => {
...

The deleteOne() deletes the document of Volkswagen.

MongoDB updateOne

The updateOne() function is used to update a document.

update_one.js

const mongo = require('mongodb');
const MongoClient = mongo.MongoClient;

const url = 'mongodb://localhost:27017';

MongoClient.connect(url, { useNewUrlParser: true }, (err, client) => {

if (err) throw err;

const db = client.db("testdb");

let filQuery = { name: "Audi" };
let updateQuery = { $set: { "price": 52000 }};

db.collection('cars').updateOne(filQuery, updateQuery).then(result => {

    console.log('Car updated');
    console.log(result);

}).catch((err) => {

    console.log(err);
}).finally(() => {

    client.close();
});

});

The example updats a prices of a car.

let filQuery = { name: "Audi" };
let updateQuery = { $set: { "price": 52000 }};

db.collection('cars').updateOne(filQuery, updateQuery).then(result => {

The price of Audi is changed to 52,000 with the updateOne() method. The $set operator is used to change the price.

> db.cars.find({name:'Audi'})
{ "_id" : ObjectId("5cfcfc3438f62aaa09b52175"), "name" : "Audi", "price" : 52000 }

We confirm the changes with the mongo tool.

In this tutorial, we have worked with MongoDB and JavaScript. Thanks For Visiting, Keep Visiting

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JavaScript Tutorial: if-else Statement in JavaScript

JavaScript Tutorial: if-else Statement in JavaScript

This JavaScript tutorial is a step by step guide on JavaScript If Else Statements. Learn how to use If Else in javascript and also JavaScript If Else Statements. if-else Statement in JavaScript. JavaScript's conditional statements: if; if-else; nested-if; if-else-if. These statements allow you to control the flow of your program's execution based upon conditions known only during run time.

Decision Making in programming is similar to decision making in real life. In programming also we face some situations where we want a certain block of code to be executed when some condition is fulfilled.
A programming language uses control statements to control the flow of execution of the program based on certain conditions. These are used to cause the flow of execution to advance and branch based on changes to the state of a program.

JavaScript’s conditional statements:

  • if
  • if-else
  • nested-if
  • if-else-if

These statements allow you to control the flow of your program’s execution based upon conditions known only during run time.

  • if: if statement is the most simple decision making statement. It is used to decide whether a certain statement or block of statements will be executed or not i.e if a certain condition is true then a block of statement is executed otherwise not.
    Syntax:
if(condition) 
{
   // Statements to execute if
   // condition is true
}

Here, condition after evaluation will be either true or false. if statement accepts boolean values – if the value is true then it will execute the block of statements under it.
If we do not provide the curly braces ‘{‘ and ‘}’ after if( condition ) then by default if statement will consider the immediate one statement to be inside its block. For example,

if(condition)
   statement1;
   statement2;

// Here if the condition is true, if block 
// will consider only statement1 to be inside 
// its block.

Flow chart:

Example:

<script type = "text/javaScript"> 

// JavaScript program to illustrate If statement 

var i = 10; 

if (i > 15) 
document.write("10 is less than 15"); 

// This statement will be executed 
// as if considers one statement by default 
document.write("I am Not in if"); 

< /script> 

Output:

I am Not in if
  • if-else: The if statement alone tells us that if a condition is true it will execute a block of statements and if the condition is false it won’t. But what if we want to do something else if the condition is false. Here comes the else statement. We can use the else statement with if statement to execute a block of code when the condition is false.
    Syntax:
if (condition)
{
    // Executes this block if
    // condition is true
}
else
{
    // Executes this block if
    // condition is false
}


Example:

<script type = "text/javaScript"> 

// JavaScript program to illustrate If-else statement 

var i = 10; 

if (i < 15) 
document.write("10 is less than 15"); 
else
document.write("I am Not in if"); 

< /script> 

Output:

i is smaller than 15
  • nested-if A nested if is an if statement that is the target of another if or else. Nested if statements means an if statement inside an if statement. Yes, JavaScript allows us to nest if statements within if statements. i.e, we can place an if statement inside another if statement.
    Syntax:
if (condition1) 
{
   // Executes when condition1 is true
   if (condition2) 
   {
      // Executes when condition2 is true
   }
}

Example:

<script type = "text/javaScript"> 

// JavaScript program to illustrate nested-if statement 

var i = 10; 

if (i == 10) { 

// First if statement 
if (i < 15) 
	document.write("i is smaller than 15"); 

// Nested - if statement 
// Will only be executed if statement above 
// it is true 
if (i < 12) 
	document.write("i is smaller than 12 too"); 
else
	document.write("i is greater than 15"); 
} 
< /script> 

Output:

i is smaller than 15
i is smaller than 12 too
  • if-else-if ladder Here, a user can decide among multiple options.The if statements are executed from the top down. As soon as one of the conditions controlling the if is true, the statement associated with that if is executed, and the rest of the ladder is bypassed. If none of the conditions is true, then the final else statement will be executed.
if (condition)
    statement;
else if (condition)
    statement;
.
.
else
    statement;


Example:

<script type = "text/javaScript"> 
// JavaScript program to illustrate nested-if statement 

var i = 20; 

if (i == 10) 
document.wrte("i is 10"); 
else if (i == 15) 
document.wrte("i is 15"); 
else if (i == 20) 
document.wrte("i is 20"); 
else
document.wrte("i is not present"); 
< /script> 

Output:

i is 20