JavaScript Operator: in
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In JavaScript,
the in operator returns
true if the specified property is in the specified object, otherwise false. The
results can be a little counterintuitive until the exact nature of the object
is taken into account:
For example, suppose we
have an object such as:
var x = {name: "ten", val: 10,
binaryval: "1010"};
Then:
|
y
|
y in x
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Explanation
|
|
"name"
|
true
|
"name" is a
valid property of the object
|
|
"val"
|
true
|
"val" is a
valid property of the object
|
|
"length"
|
true
|
The object has a length
(number of entries)
|
|
1
|
false
|
An object is not
indexed like an array, so there is no entry indexed by 1
|
Suppose, instead, we have
an array such as:
var x = ["a","b"];
Then
|
y
|
y in x
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Explanation
|
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"a"
|
false
|
"a" is not a
valid property, rather it is the value assigned to the property (the property
is the index number)
|
|
"length"
|
true
|
As arrays do
generically have a length property
|
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1
|
true
|
As there is an entry
with index number 1 (the entry is "b"), although if y were 2 then
it would be false (as there is no entry with index number 2)
|
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