Pass the i modifier as second argument:

new RegExp('^' + query, 'i');

Have a look at the documentation for more information.

One way is to build an regex based on offset and pattern passed as string

• Here initial part of regex uses . to match any character upto offset
• Second use passed pattern in captured group to return only desired match

let startsWith = (str, reg, offset) =>{
  let regex = `^.{${0,offset}}(${reg})`
  let final = new RegExp(regex,'i')
  let found = str.match(final)
  console.log(found ? found[1] : found)
}

startsWith("Hello world !", 'h', 0); // "H"
startsWith("Hello world !", 'wor?', 6); // "wor"
startsWith("Hello world !", 'wor?', 10); // null
startsWith("Hello world !", 'z', 0); // null

As mentioned by @mark we can use source property to if we want regex to be passed as argument to function

let startsWith = (str, reg, offset) =>{
  let regex = `^.{${0,offset}}(${reg.source})`
  let final = new RegExp(regex,'i')
  let found = str.match(final)
  console.log(found ? found[1] : found)
}

startsWith("Hello world !", /h/i, 0); // "H"
startsWith("Hello world !", /wor?/i, 6); // "wor"
startsWith("Hello world !", /wor?/i, 10); // null
startsWith("Hello world !", /z/i, 0); // null

If you care about case sensitivity then you could use a regex

var isValidImage = /^image/i.test(inputType);

If not then you can just use startsWith, but keep in mind browser compatibility. You might need a Polyfill.

It is simpler to just use the regex which is compatible everywhere.