I like Pablo's answer, but Array#indexOf and Array#map don't work on all browsers. Underscore will use native code if it's available, but has fallbacks as well. More on stackoverflow.com

Array.prototype.findIndex takes a function as its argument. That's right, functions themselves can be passed into functions! First, we can define a function that returns true if the uniqueId is 2 and false otherwise. function uniqueIdIsTwo(user) { return user.uniqueId === 2 } Then we can pass that function as the parameter of findIndex const index = myArray.findIndex(uniqueIdIsTwo) Notice how we're passing the function itself, we're not calling it! findIndex takes care of calling our function. In fact, findIndex calls our function for every element of the array and comes back with the index when our function returns true (or returns the magical -1 if our function never returns true). It's annoying to define a separate function each time, so we can also use an anonymous function instead. Anonymous means it doesn't have a name. Naming things is hard, so this is a useful thing to avoid naming it! const index = myArray.findIndex(function(user) { return user.uniqueId === 2}) Finally, we can also use arrow functions, which are a little more concise. There are differences between arrow functions and "regular" functions that are important to learn (some other time) but they are not different for this particular case. const index = myArray.findIndex(user => user.uniqueId === 2) More on reddit.com

It's not a terribly common scenario. If you want data to remain sorted and you cared about performance, you would normally leverage a sorted data structure (there are various trees structures that support this), rather than a dynamic array. Inserting in the middle of a dynamic array is a very inefficient operation in the worst case O(n) in the length of the array, and so is usually only a good idea on relatively small arrays. And if your array is small, might as well just append at the end and sort it, or just use a simple linear scan on the array using index of. Those few cycles won't matter conpared to network overhead anyway. Fortunately almost all arrays you will work with day to day are small. This is the kind of task that feels common in leetcode, but is rareish in production use cases. More on reddit.com

Think about what your if statement is doing. If the array element does not match then immediately return false. Do you see why that may be a problem? As a side note I would stick to only returning a single type from a function. Returning an integer or a boolean can trip you up later. More on reddit.com