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

The simplest and easiest way to find element index in array.

ES5 syntax: [{id:1},{id:2},{id:3},{id:4}].findIndex(function(obj){return obj.id == 3})

ES6 syntax: [{id:1},{id:2},{id:3},{id:4}].findIndex(obj => obj.id == 3)

The array.findIndex() function will bring back the index for the first value that the comparing function results are true. I have a function that I use to compare if two objects are equal that can be used in this example.

var obj = {a: 1, b: 2};
var arr = [];
arr[0] = {a: 1, b: 2};
arr[1] = {a: 1, b: 3};

var idx = arr.findIndex(element => areObjsEqual(element,obj));
console.log(`The index is: ${idx}`);

//Function to check if two js objects are equal
function areObjsEqual(a, b) {
    // Create arrays of property names
    var aProps = Object.getOwnPropertyNames(a);
    var bProps = Object.getOwnPropertyNames(b);

    // If number of properties is different,
    // objects are not equivalent
    if (aProps.length != bProps.length) { return false;}
    
    //loop through the object and compare the property values
    for (var i = 0; i < aProps.length; i++) {
        var propName = aProps[i];
        // If values of same property are not equal,
        // objects are not equivalent
        if (a[propName] !== b[propName]) {
            return false;
        }
    }
    // If we made it this far, objects
    // are considered equivalent
    return true;
}