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What is type conversion in JavaScript?

Ayush Trivedi

Grokking Modern System Design Interview for Engineers & Managers

Ace your System Design Interview and take your career to the next level. Learn to handle the design of applications like Netflix, Quora, Facebook, Uber, and many more in a 45-min interview. Learn the RESHADED framework for architecting web-scale applications by determining requirements, constraints, and assumptions before diving into a step-by-step design process.

When the type of a variable is changed to another type, it is known as type conversion or type casting.

JavaScript does not have float, Int, double, or long types like other languages such as C, C++, and Java. Instead, it has a common type for all numerical values: Number. Hence, type conversion is needed so that the right operation can be formed for a certain variable.

How to convert a string to a number

Let’s convert a String type variable to a Number type variable. We will also see that performing the add (+) operation has a different effect on variables before conversion and after conversion.

To convert a String variable into a Number, we will use the JavaScript global method Number().

Parameters

Number() takes one parameter of string type.

Return value

Number() will return a value of Number type, or NaN if the string is not valid or does not contain a numeric value.

Code

var a = "3.12";

console.log(typeof(a));

console.log(a + 3);

var a = Number(a);

console.log(typeof(a), " value ", a + 3);

The example above shows how to do a type conversion. It’s a simple but useful thing to know while working code.

Explanation

  • In line 1, we declare a variable of string type. Writing a character in quotes (""/'') will treat the variable as string.

  • In line 3, we check the type of variable using the typeOf() method and print it using console.log().

  • In line 5, we perform an add operation on the variable.

  • In line 7, we convert the string to Number using global method Number().

  • In line 9, we again check the type of variable a and perform an add operation on it, and then print it on console.

Output

As we can see, the first line of output indicates that the variable was a string. When we perform the add operation it, we append the number 3 to the string 3.12.

But the result we wanted was 6.12. After using the global method, Number(), the result we get in the last line is Number. This means we have successfully achieved type conversion, and the result we wanted, 6.12, is returned.

What if an invalid string is passed in Number()?

If an invalid string, a string which contains alphabets or symbols, is passed to Number(), then the method will convert it to NaN (Not a Number).

The code below is an example showing if the string passed is not a valid number.

var a = "33 asd 12";

var b = Number(a);

console.log(b);

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javascript
communitycreator

Grokking Modern System Design Interview for Engineers & Managers

Ace your System Design Interview and take your career to the next level. Learn to handle the design of applications like Netflix, Quora, Facebook, Uber, and many more in a 45-min interview. Learn the RESHADED framework for architecting web-scale applications by determining requirements, constraints, and assumptions before diving into a step-by-step design process.

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