In setCustomFormats method of src/component.js:
setCustomFormats: function (editor) {
if (!this.props.formats) {
return;
}
for (var i = 0; i < this.props.formats.length; i++) {
var format = this.props.formats[i];
editor.addFormat(format.name || format, format);
}
},
this method calls editor.addFormat, but it seems that this function is no longer supported in Quill 1.0.