Array.prototype.findIndex is supported in all browsers other than IE (non-edge). But the polyfill provided is nice.

var indexOfStevie = myArray.findIndex(i => i.hello === "stevie");

The solution with map is okay. But you are iterating over the entire array every search. That is only the worst case for findIndex which stops iterating once a match is found.

var searchTerm = "stevie",
    index = -1;
for(var i = 0, len = myArray.length; i < len; i++) {
    if (myArray[i].hello === searchTerm) {
        index = i;
        break;
    }
}

or as a function:

function arrayObjectIndexOf(myArray, searchTerm, property) {
    for(var i = 0, len = myArray.length; i < len; i++) {
        if (myArray[i][property] === searchTerm) return i;
    }
    return -1;
}
arrayObjectIndexOf(arr, "stevie", "hello"); // 1

Just some notes:

1. Don't use for...in loops on arrays
2. Be sure to break out of the loop or return out of the function once you've found your "needle"
3. Be careful with object equality

For example,

var a = {obj: 0};
var b = [a];
b.indexOf({obj: 0}); // -1 not found