How to find the array index with a value?

Use jQuery's function jQuery.inArray

jQuery.inArray( value, array [, fromIndex ] )
(or) $.inArray( value, array [, fromIndex ] )

You can use indexOf:

var imageList = [100,200,300,400,500];
var index = imageList.indexOf(200); // 1

You will get -1 if it cannot find a value in the array.

For objects array use map with indexOf:

var imageList = [
   {value: 100},
   {value: 200},
   {value: 300},
   {value: 400},
   {value: 500}
];

var index = imageList.map(function (img) { return img.value; }).indexOf(200);

console.log(index);

In modern browsers you can use findIndex:

var imageList = [
   {value: 100},
   {value: 200},
   {value: 300},
   {value: 400},
   {value: 500}
];

var index = imageList.findIndex(img => img.value === 200);

console.log(index);

Its part of ES6 and supported by Chrome, FF, Safari and Edge