javascript indexOf strange behavior

Question

I found a strange unexplainable behavior while writing a piece of test code for my javascript program. I was comparing an output of res.body to check if it contains a string.

More precisely, I was checking if res.body contained a string 'channel'.

Even though the output did really contain that string, the test case was always failing. I ran the program in debug mode, to examine why it is failing. The results are puzzling, to say the least.

> res.body
'Error: invalid channnel'
> res.body.indexOf('channel')
-1
> res.body.indexOf('channe')
-1
> res.body.indexOf('chann')
15
> 

if I try to check indexOf of 'channel' in res.body whose value was 'Error: invalid channel', I was supposed to get a positive value, but it gives -1.

so, I tried by reducing the no. of chars in the match, it still fails until I reduce the search string to contain just 'chann' ie by omitting 'el'.

But this behavior doesn't happen, if I take a string litteral and do the same exercise. For example, the below works perfectly.

> 'Error: invalid channel'.indexOf('channel')
15

I went ahead and checked the typeof res.body, and it shows as string, and not any object.

> typeof res.body
'string'
> 

Did anyone face a similar problem ever? Or is there an explanation to this?

EDIT It happens when I res.body as produced by mocha + chai framework. The full code is below:

 58   it ("Should fail with 400, if invalid channel", function(done) {
 59     chai.request(app).post('/campaigns/js')
 60     .send(samples.type0.invalidChannel)
 61     .end(function(err, res) {
 62       console.log("response:", res.body);
 63       expect(res).to.have.status(400);
>64       debugger;
 65       expect(res.body).to.contain('channel');
 66       return done();
 67     });
 68   });

Answer 1

You're res.body has channel with 3 n s.

'Error: invalid channnel'

This means that chann matches, but channe won't because of the extra n .

Answer 2

In the first piece of code, "channnel" has 3 n's. That's why it matched "chann" but not "channe".