Angular js 1.7 $onInit function syntax

I recently updated my app from angularjs 1.5.9 to angularjs 1.7.9. In one of my controllers I had the following code:

angular.module( 'myModule' )
.component( 'dynamicContent', {
    bindings: {
        pageTitle: '<',
        content: '<',
        type: '<',
        width: '<'
    },
    templateUrl: 'path/to/html/file',
    controllerAs: 'dynamic',
    controller: function( $state, $window, Contentful ) {
        var vm = this;

        vm.$onInit = _init();

        function _init() {
            vm.items = _.map( vm.content, mapContent );
        }

After updating to 1.7.9 the vm.items variable was never getting defined because the vm.content was undefined in the _init function. However, after changing the code to...

 angular.module( 'myModule' )
.component( 'dynamicContent', {
    bindings: {
        pageTitle: '<',
        content: '<',
        type: '<',
        width: '<'
    },
    templateUrl: 'path/to/html/file',
    controllerAs: 'dynamic',
    controller: function( $state, $window, Contentful ) {
        var vm = this;

        vm.$onInit = function _init() {
            vm.items = _.map( vm.content, mapContent );
        }

This now works. Can anyone explain why defining the function where I set it to $onInit makes this work? Thanks!

vm.$onInit = _init();

This is equal to simple _init();, you probably meant vm.$onInit = _init.

In 1.5 you can run init yourself and there was no real difference, but in latest Angularjs bindings are set after your controller main function, but before on init, e.g.:

bindings: { test: '<'}
controller: () => {
  var vm = this;
  console.log(vm.test); // undefined
  vm.$onInit = () => {
    console.log(vm.test); // has value
  }
}