To show variable type use typeof:
let value = true; console.log(typeof value); // boolean
Let’s convert our variable to string:
value = String(value); // now value is a string "true" console.log(typeof value); // string
Numeric Conversion
Numeric conversion happens in mathematical functions and expressions automatically.
For example, when division / is applied to non-numbers:
console.log( "6" / "2" ); // 3, strings are converted to numbers
We can use the Number(value) function to explicitly convert a value to a number:
let str = "123"; console.log(typeof str); // string let num = Number(str); // becomes a number 123 console.log(typeof num); // number
Explicit conversion is usually required when we read a value from a string-based source like a text form but expect a number to be entered.
If the string is not a valid number, the result of such a conversion is NaN. For instance:
let age = Number("an arbitrary string instead of a number");
console.log(age); // NaN, conversion failed
Examples of numeric conversion:
console.log( Number(" 123 ") ); // 123
console.log( Number("123z") ); // NaN (error reading a number at "z")
console.log( Number(true) ); // 1
console.log( Number(false) ); // 0
Please note that null and undefined behave differently here: null becomes zero while undefined becomes NaN.
Boolean Conversion
The conversion rule:
Values that are intuitively “empty”, like 0, an empty string, null, undefined, and NaN, become false.
Other values become true.
For instance:
console.log( Boolean(1) ); // true
console.log( Boolean(0) ); // false
console.log( Boolean("hello") ); // true
console.log( Boolean("") ); // false
Summary
The three most widely used type conversions are to string, to number, and to boolean.
String Conversion – Occurs when we output something. Can be performed with String(value). The conversion to string is usually obvious for primitive values.
Numeric Conversion– Occurs in math operations. Can be performed with Number(value).
Boolean Conversion – Occurs in logical operations. Can be performed with Boolean(value).