When is the index of array compared with -1

Question

I have seen multiple codes where index of arrays are compared to -1, like

if (index === -1)

What does this exactly do?

I believe we can't store values in negative index, at least as array element. (I know, As a property of array, we can indeed do that). But then when will the above condition return true or false?

As addition to @Joseph the Dreamer answer: for "beauty" comparison sometimes used bitwise operator '~'. It equals expression -(n+1). So you can write it like if ( ~str.indexOf(...) ) { console.log('Found!') }. — Salasar
– Salasar, Commented Sep 15, 2015 at 13:01

Joseph · Accepted Answer · 2015-09-15 12:48:37Z

12

This is usually used with an array's or string's indexOf operation. indexOf returns a -1 when the value is not in the array. Otherwise, it returns a zero-based index of the first match it finds.

The condition you have above probably checks if an item does not exist in the array.

answered Sep 15, 2015 at 12:48

Joseph

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2 Comments

T.J. Crowder Over a year ago

And yes, before anyone asks, it is reliably -1, not just a negative number, for Array#indexOf and String#indexOf (and Array#findIndex). Naturally YMMV with other functions, check their specification.

Sergey Over a year ago

... and -1 was specifically chosen because, as OP mentions, "we can't store values in negative index", so it's suited for the "not found" value.

Aefits · Accepted Answer · 2015-09-16 07:17:00Z

0

consider below code

var fruits = ["Banana", "Orange", "Apple", "Mango"]; var index = fruits.indexOf("Apple"); if(index != -1) { alert ("Apple found at " + index) } else { alert ("Apple not in exists") }

array indexof returns index number(0 or 1 or 2) of the value if the value exists but returns -1 if the value does not exists.

thanks!

edited Sep 16, 2015 at 7:17

answered Sep 15, 2015 at 12:53

Aefits

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4 Comments

T.J. Crowder Over a year ago

Think you have that comparison backward. In any case, Joseph already covered this ground several minutes ago.

Aefits Over a year ago

Did not see his reply when I started typing my answer and it took some time when i was typing my answer and Joseph was little faster than me.

Saurabh Tiwari Over a year ago

@Shahid index value in your code will be 2. Why should Apple found at + index be shown when index=== -1 be true

Aefits Over a year ago

@Suarabh - I have corrected the if condition thanks for pointing out.

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