Javascript: indexOf always returns -1, but not for all arrays -

i'm using vue.js. have 2 arrays, checkedtracks , checkedalbums. both created , populated in same way (see initialisealbums) , both contain elements of type string.

the updatearray method takes array , array element parameters. if element in array, removed. if not in array, added. when call method updatearray , pass checkedtracks array, performs expected. when pass checkedalbums, index -1.

while debugging method checkedalbums noticed few things.

• the data types of element being passed in , array elements same
• the array contains data, array[0] returns undefined
• manually passing id string indexof function still returns 0, despite being identical

can provide insight going on here? in advance.

edit: have simplified code snippet include relevant information

<script> export default {   data () {     return {       allalbums: [],       checkedtracks: [],       checkedalbums: []     }   },   methods: {     initialisealbums: function (allalbums) {       let self =       self.allalbums = []       self.checkedalbums = []       self.checkedtracks = []       allalbums.foreach(function (album) {         if (album.tracks.items !== null) {           self.allalbums.push(album)           self.checkedalbums.push(album.id)           album.tracks.items.foreach(function (track) {             self.checkedtracks.push(track.id)           })         }       })     },     togglesinglealbum: function (albumid) {       this.updatearray(this.checkedalbums, albumid)     },     updatetrack: function (trackid) {       this.updatearray(this.checkedtracks, trackid)     },     updatearray: function (array, element) {       let index = array.indexof(element)       if (index > -1) {         array.splice(index, 1)       } else {         array.push(element)       }     }   } } </script>