JS Fundamentals - Type Conversions

Question 1

Type Conversion

Answer 1

Most of the time, operators and functions automatically convert the values given to them to the right type.

For example, alert automatically converts any value to a string to show it. Mathematical operations convert values to numbers.

There are also cases when we need to explicitly convert a value to the expected type.

Question 2

String conversion

Answer 2

We can also call the String(value) function to convert a value to a string:

Question 3

Example of String Conversion

Answer 3

let value = true;
alert(typeof value); // boolean

value = String(value); // now value is a string “true”
alert(typeof value); // string

String conversion is mostly obvious. A false becomes “false”, null becomes “null”, etc.

Question 4

Numeric Conversion

Answer 4

alert( "6" / "2" ); // 3, strings are converted to numbers
let str = "123";
alert(typeof str); // string

let num = Number(str); // becomes a number 123

alert(typeof num); // number

Question 5

When does NaN happen in Numeric conversion

Answer 5

let age = Number(“an arbitrary string instead of a number”);

alert(age); // NaN, conversion failed

Question 6

Examples of Numeric Conversion

Answer 6

Value	Becomes…
undefined	NaN
null	                0
true and false	1 and 0
string	Whitespaces from the start and end are removed. If the remaining string is empty, the result is 0. Otherwise, the number is “read” from the string. An error gives NaN.

Question 7

null and undefined

Answer 7

Please note that null and undefined behave differently here: null becomes zero while undefined becomes NaN.

Question 8

Boolean Conversion

Answer 8

It happens in logical operations (later we’ll meet condition tests and other similar things) but can also be performed explicitly with a call to Boolean(value).

The conversion rule:

Values that are intuitively “empty”, like 0, an empty string, null, undefined, and NaN, become false.
Other values become true.

Question 9

Please Note

Answer 9

Some languages (namely PHP) treat “0” as false. But in JavaScript, a non-empty string is always true.

alert( Boolean(“0”) ); // true
alert( Boolean(“ “) ); // spaces, also true (any non-empty string is true)