a: AA[];
b: AA;
ngOnInit() {
this.a = [];
}
x() {
this.a = [...this.a, this.b]; <--- ?
}
a: AA[];
b: AA;
ngOnInit() {
this.a = [];
}
x() {
this.a = [...this.a, this.b]; <--- ?
} 
I hope I can help the subject!
babel this.a = [...this.a, this.b];
this.a = [].concat(_toConsumableArray(this.a), [this.b]);from this function we can see that if a new array clone is created for an array, otherwise the Array.from method will be called to create a new array instance from a similar array or iterable object
function _toConsumableArray(arr) {
if (Array.isArray(arr)) {
for (var i = 0, arr2 = Array(arr.length); i < arr.length; iPP) {
arr2[i] = arr[i]; } return arr2;
} else {
return Array.from(arr);
}
} [...'test',10]; // ['t','e','s','t',10]
//
this.a = [...this.a, this.b];
this.a -> => this.a [].concat() this.b you can go in and have a look at specific examples.
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