Return index value from filter method javascript

You can't return index from filter method.

The filter() method creates a new array with all elements that pass the test implemented by the provided function.

You can use forEach

$scope.indexOfField = function(fieldId) {
    var i;
    return $scope.model.fieldData.forEach(function(x, index) {
        if (x.Id === fieldId) {
            i = index;
        }
    });
    // use i
}

or even better to use for as you can't stop forEach when you have found your id.

$scope.indexOfField = function(fieldId) {
    var fieldData = $scope.model.fieldData, 
        i = 0, ii = $scope.model.fieldData.length;
    for(i; i < ii; i++) if(fieldData[i].Id === fieldId) break;
    // use i
}

The .findIndex() method returns the index of the first element of the array that satisfies a condition given by a function. If the function returns false for all elements of the array, the result is -1.

See the documentation here.

In my example, x is an item for each iteration and I use cross function for my condition.

const datas = [];
const fieldId = 5;
let index = datas.findIndex( x => x.Id === fieldId );