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Today I tried shortening an if statement like this:

if (fruit == "apple" || type == "pear" || type == "orange"){

to this:

if (fruit in ["apple", "pear", "orange"]){

It doesn't work. Jonathan Snook has a different solution here where he essentially uses a map like this, which does work:

if(fruit in {"apple":"", "pear":"", "orange":""}){

Why does that work and my simple array doesn't, when usually in Javascript an object's presence makes that object return true? Is a string a different type of object than a key? I thought the string's presence in my array would return true as well.

y = "Stackoverflow";
y && console.log('y is true') // y returns true, so console logs the message

I interpreted my original solution as "if the value of variable fruit is in this array." That's clearly not the case. Does it instead say "if the variable fruit itself is in this array?" No, because this doesn't work either:

if (fruit in [fruit, "apple", "pear", "orange"]){

So what is Snook's version with the key=>value map asking that's correct? My best guess is, "if a key under the name of the value of variable fruit in this map returns true?"

Artur Sapek
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2 Answers2

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x in y returns true if the object y has a property with name x (so yes, your guess is correct).

Arrays are objects too. The properties of an array are the indexes, which are numerical †. This works:

if(0 in ["apple", "pear", "orange"])

because the array has an element at index 0. This array is similar (but not the same!) to this object:

{0: "apple", 1: "pear", 2:"orange"}

(of course an array has further properties like length, push, slice, etc)

In your object example ({"apple":"", "pear":"", "orange":""}), apple, pear etc. are the properties of the object, not the values of properties.

How to find out whether an element is contained in an array is described in How do I check if an array includes an object in JavaScript? .

†: Strictly speaking, every property is a string, so even if you use numbers (as it is the case with arrays), they are converted to strings.

Community
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Felix Kling
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  • Right, but I'm trying to check the value of a variable against the strings in the array. Your statement will always be true, but `fruit` might not be apple, pear, or orange. – Artur Sapek Aug 28 '11 at 21:43
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    I don't get your point. You wanted to know why it does not work and I answered that. You can replace `0` with `5` and it will return `false`. – Felix Kling Aug 28 '11 at 21:46
  • Yeah I get it, I thought you were suggesting your code as an alternative to mine, but I see now you were just making a point. Thanks – Artur Sapek Aug 28 '11 at 21:47
  • Ah ok, no, I did not suggest this as alternative (you did not ask for it ;)) And if you did, I'm pretty sure it was already asked. – Felix Kling Aug 28 '11 at 21:49
  • Yeah I know about `indexOf`. Thank you for your time. The array to map comparison helped me understand – Artur Sapek Aug 28 '11 at 21:52
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To check if a value is within an array, use indexOf:

if (["apple", "pear", "orange"].indexOf(fruit) != -1){

Note: IE < 9 does not support indexOf on arrays, but you can add support easily.

Joseph Silber
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