Jquery how to find an Object by attribute in an Array

you should pass reference on item in grep function:

function findPurpose(purposeName){
    return $.grep(purposeObjects, function(item){
      return item.purpose == purposeName;
    });
};

Example

No need for jQuery.

JavaScript arrays have a find method, so you can achieve that in one line:

array.find((o) => { return o[propertyName] === propertyValue })

Example

const purposeObjects = [
    {purpose: "daily"},
    {purpose: "weekly"},
    {purpose: "monthly"}
];

purposeObjects.find((o) => { return o["purpose"] === "weekly" })      

// output -> {purpose: "weekly"}

If you need IE compatibility, import this polyfill in your code.

I personally use a more generic function that works for any property of any array:

function lookup(array, prop, value) {
    for (var i = 0, len = array.length; i < len; i++)
        if (array[i] && array[i][prop] === value) return array[i];
}

You just call it like this:

lookup(purposeObjects, "purpose", "daily");