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.
In this chapter, we won’t cover objects. For now, we’ll just be talking about primitives.
Later, after we learn about objects, in the chapter Object to primitive conversion we’ll see how objects fit in.
String Conversion
String conversion happens when we need the string form of a value.
For example, alert(value)
does it to show the value.
We can also call the String(value)
function to convert a value to a string:
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.
Numeric Conversion
Numeric conversion in mathematical functions and expressions happens automatically.
For example, when division /
is applied to non-numbers:
alert( "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";
alert(typeof str); // string
let num = Number(str); // becomes a number 123
alert(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");
alert(age); // NaN, conversion failed
Numeric conversion rules:
Value | Becomes… |
---|---|
undefined |
NaN |
null |
0 |
true and false |
1 and 0 |
string |
Whitespaces (includes spaces, tabs \t , newlines \n etc.) 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 . |
Examples:
alert( Number(" 123 ") ); // 123
alert( Number("123z") ); // NaN (error reading a number at "z")
alert( Number(true) ); // 1
alert( Number(false) ); // 0
Please note that null
and undefined
behave differently here: null
becomes zero while undefined
becomes NaN
.
Most mathematical operators also perform such conversion, we’ll see that in the next chapter.
Boolean Conversion
Boolean conversion is the simplest one.
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
, andNaN
, becomefalse
. - Other values become
true
.
For instance:
alert( Boolean(1) ); // true
alert( Boolean(0) ); // false
alert( Boolean("hello") ); // true
alert( Boolean("") ); // false
"0"
is true
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)
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)
.
The conversion follows the rules:
Value | Becomes… |
---|---|
undefined |
NaN |
null |
0 |
true / false |
1 / 0 |
string |
The string is read “as is”, whitespaces (includes spaces, tabs \t , newlines \n etc.) from both sides are ignored. An empty string becomes 0 . An error gives NaN . |
Boolean Conversion
– Occurs in logical operations. Can be performed with Boolean(value)
.
Follows the rules:
Value | Becomes… |
---|---|
0 , null , undefined , NaN , "" |
false |
any other value | true |
Most of these rules are easy to understand and memorize. The notable exceptions where people usually make mistakes are:
undefined
isNaN
as a number, not0
."0"
and space-only strings like" "
are true as a boolean.
Objects aren’t covered here. We’ll return to them later in the chapter Object to primitive conversion that is devoted exclusively to objects after we learn more basic things about JavaScript.
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