TypeScript: Cannot read property 'push' of undefined in [null]

Question

Error: Cannot read property 'push' of undefined in [null].

class A implements OnInit {
    stringArr: string[];

    ngOnInit() {
        for(let i=0; i<4; i++) {
            this.stringArr.push("aaa");
        }
    }
}

Answer 1

The array needs to be initialized:

stringArr = [];

Answer 2

For me the syntax was a little more than the accepted answer because I was using a full array type.

stringArr: Array<string> = [];

Also, this would work too

stringArr: string[] = [];

Just 2 ways to type a new array

Answer 3

class A implements OnInit {
    stringArr: string=[];
    ngOnInit() {
        for(let i=0; i<4; i++) {
            this.stringArr.push("aaa");
        }
    }
}

Here stringArr is defined as string, but not as an array of string. Define stringArr as blank array [].