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Subtract

Just like the multiplicative operators, additive operators also behave in special ways when dealing with
special values. If the two operands are numbers, they perform an arithmetic add and return the result
according to these rules:
?
If either number is
NaN
, the result is
NaN
.
?
If
Infinity
Infinity
, the result is
Infinity
.
?
If –
Infinity
Infinity
, the result is –
Infinity
.
?
If
Infinity
Infinity
, the result is
NaN
.
?
If +0 is added to +0, the result is +0.
?
If –0 is added to +0, the result is +0.
?
If –0 is added to –0, the result is –0.
If, however, one of the operands is a string, then the following rules are applied:
?
If both operands are strings, the second string is concatenated to the first.
?
If only one operand is a string, the other operand is converted to a string and the result is the
concatenation of the two strings.
For example:
var result1 = 5 + 5; //two numbers
var result2 = 5 + “5”; //a number and a string
This code illustrates the difference between the two modes for the add operator. Normally,
5 + 5
equals
10
(a primitive number value), as illustrated by the first two lines of code. However, if one of the operands
is changed to a string,
“5”
, the result becomes
“55”
(which is a primitive string value) because the first
operand gets translated to
“5”
as well.
Subtract
The subtract operator (–) is another that is used quite frequently:
var iResult = 2 – 1;
Just like the add operator, the subtract operator has special rules to deal with the variety of type conver-
sions present in ECMAScript:
?
If the two operands are numbers, perform arithmetic subtract and return the result.
?
If either number is
NaN
, the result is
NaN
.
To avoid one of the most common mistakes made in JavaScript, always double check
the data types when using the add operator.
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Chapter 2
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