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Equality operators

var bResult = “23” < 3;

alert(bResult); //outputs “false”

Here, the string

“23”

is converted into the number

and then compared to

, giving the expected

result. Whenever a number is compared to a string, ECMAScript says that the string should be con-

verted into a number and then numerically compared with the other number. This works well for

cases like the previous example, but what if the string can’t be converted into a number? Consider this

example:

var bResult = “a” < 3;

alert(bResult);

What would you expect this to output? The letter

“a”

can’t be meaningfully converted into a number.

After all, if you were to use

parseInt()

on it,

NaN

would be returned. As a rule, any relational opera-

tion that contains

NaN

returns

false

, so this code also outputs

false

var bResult = “a” >= 3;

alert(bResult);

Typically, if two values return

false

for a less-than operation, they must return

true

for a greater-than-

or-equal operation, but this is not the case when one number is

NaN

Equality operators

Determining whether two variables are equivalent is one of the most important operations in program-

ming. This is fairly straightforward when dealing with primitive values, but the task gets a little compli-

cated when you take objects into account. To deal with this problem, ECMAScript provides two sets of

operators: equal and not equal to deal with primitive values, and identically equal and not identically

equal to deal with objects.

Equal and not equal

The equal operator in ECMAScript is the double equal sign (

), and it returns

true

if — and only if —

both operands are equal. The not equal operator is the exclamation point followed by an equal sign (

and it returns

true

if — and only if — two operands are not equal. Both operators do conversions in

order to determine if two operands are equal.

When performing conversions, follow these basic rules:

If an operand is a Boolean value, convert it into a numeric value before checking for equality.

A value of

false

converts to 0; whereas a value of

true

converts to 1.

If one operand is a string and the other is a number, attempt to convert the string into a number

before checking for equality.

If one operand is an object and the other is a string, attempt to convert the object to a string

(using the

toString()

method) before checking for equality.

If one operand is an object and the other is a number, attempt to convert the object to a number

before checking for equality.

Chapter 2

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