Checking Data Type and Type Conversion

 

JavaScript Type Conversions and Checking Data type

• To check the data type of a certain variable we use the typeof method.
• Type conversion is the process of converting data of one type to another

example: checking data type.

let firstName = 'naveen'      // string

console.log(typeof firstName)   // string

There are two types of type conversion in JavaScript.

• Implicit Conversion - automatic type conversion
• Explicit Conversion - manual type conversion

JavaScript Implicit Conversion

• JavaScript automatically converts one data type to another.
• This is known as implicit conversion.

Examples

Implicit Conversion to String

// numeric string used with + gives string type

let result;

result = '3' + 2;

console.log(result) // "32"

result = '3' + true;

console.log(result); // "3true"

result = '3' + undefined;

console.log(result); // "3undefined"

result = '3' + null;

console.log(result); // "3null"

• When a number is added to a string, JavaScript converts the number to a string before concatenation.

Implicit Conversion to Number

let num;

num = '4' - '2';

console.log(num); // 2

num = '4' - 2;

console.log(num); // 2

num = '4' * 2;

console.log(num); // 8

num = '4' / 2;

console.log(num); // 2

Non-numeric String Results to NaN

let str;

str = 'hello' - 'world';

console.log(str); // NaN

str = '4' - 'hello';

console.log(str); // NaN

Implicit Boolean Conversion to Number

• if boolean is used, true is 1, false is 0

let bol;

bol = '4' - true;

console.log(bol); // 3

bol = 4 + true;

console.log(bol); // 5

bol = 4 + false;

console.log(bol); // 4

• JavaScript considers 0 as false and all non-zero number as true. And, if true is converted to a number, the result is always 1.

null Conversion to Number

let nul;

nul = 4 + null;

console.log(nul);  // 4

nul = 4 - null;

console.log(nul);  // 4

undefined used with number, boolean, or null

• Arithmetic operation of undefined with number, boolean, or null gives NaN

let und;

und = 4 + undefined;

console.log(und);  // NaN

und = 4 - undefined;

console.log(und);  // NaN

und = true + undefined;

console.log(und);  // NaN

und = null + undefined;

console.log(und);  // NaN

JavaScript Explicit Conversion

• You can also convert one data type to another as per your needs
• The type conversion that you do manually is known as explicit type conversion.
• explicit type conversions are done using built-in methods
• Here are some common methods of explicit conversions

Convert to Number Explicitly

• To convert numeric strings and boolean values to numbers, you can use Number().

let numexplict;

// string to number

numexplict = Number('324');

console.log(numexplict); // 324

numexplict = Number('324e-1')

console.log(numexplict); // 32.4

// boolean to number

numexplict = Number(true);

console.log(numexplict); // 1

numexplict = Number(false);

console.log(numexplict); // 0

numexplict = Number(null);

console.log(numexplict);  // 0

numexplict = Number(' ')

console.log(numexplict);  // 0

Convert to String Explicitly

• To convert other data types to strings, you can use either String() or toString()

let strg;

strg = String(324);

console.log(strg);  // "324"

strg = String(2 + 4);

console.log(strg); // "6"

//other data types to string

strg = String(null);

console.log(strg); // "null"

strg = String(undefined);

console.log(strg); // "undefined"

• String() takes null and undefined and converts them to string. However, toString() gives an error when null is passed.

Convert to Boolean Explicitly

• To convert other data types to a boolean, you can use Boolean().

• In JavaScript, undefined, null, 0, NaN, '' converts to false

let bolexplict;

bolexplict = Boolean('');

console.log(bolexplict); // false

bolexplict = Boolean(0);

console.log(bolexplict); // false

bolexplict = Boolean(undefined);

console.log(bolexplict); // false

bolexplict = Boolean(null);

console.log(bolexplict); // false

bolexplict = Boolean(NaN);

console.log(bolexplict); // false