'in' operator in JavaScript. String comparison

Question

Hi I'm new in JavaScript and i find a basic problem:

When I use that piece of code in Python:

'a' in 'aaa'

I get True

When I do the same in JavaScript I get Error:

TypeError: Cannot use 'in' operator to search for 'a' in aaa

How to get similar result as in Python?

Another common point of confusion is the `is` operator in Python which means something totally different in C#. Python `is` means object identity comparison, C# `is` is like Python `isinstance`. — Kos, May 14 '15 at 08:11
As a general rule, javascript and Python are quite different; take care with comparison (e.g. use === in javascript), arrays (don't use `in` to go through a javascript array), dictionaries and objects (separate in Python, same thing in javascript), types (javascript is dynamically and loosely typed, Python is dynamically but strongly typed) and so on. — Phil H, May 14 '15 at 08:14

score 19 · Accepted Answer · answered May 14 '15 at 08:08

19

I think one way is to use String.indexOf()

'aaa' .indexOf('a') > -1

In javascript the in operator is used to check whether an object has a property

answered May 14 '15 at 08:08

Arun P Johny

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score 5 · Answer 2 · answered May 14 '15 at 08:09

You're looking for indexOf.

'aaa'.indexOf('a') == 0 //if a char exists in the string, indexOf will return
                        // the index of the first instance of the char
'aaa'.indexOf('b') == -1 //if a char doesn't exist in the string, indexOf will return -1

score 2 · Answer 3 · edited May 23 '17 at 12:10

2

Duplicate (How to check whether a string contains a substring in JavaScript?)

Try this:

var s = "aaaabbbaaa";
var result = s.indexOf("a") > -1;

edited May 23 '17 at 12:10

Community

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answered May 14 '15 at 08:08

suvroc

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score 2 · Answer 4 · answered May 14 '15 at 08:08

2

From MDN:

The in operator returns true if the specified property is in the specified object.

You're interested in 'aaa'.indexOf('a').

answered May 14 '15 at 08:08

Kos

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score 2 · Answer 5 · answered May 14 '15 at 08:10

2

try:

if('aaa'.search('a')>-1){
   //
}

answered May 14 '15 at 08:10

Yangguang

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you can use includes() like if('aaa'.includes('a')) – sandeep jain Mar 28 '20 at 07:32

'in' operator in JavaScript. String comparison

5 Answers5