Correct way to push into state array

Using es6 it can be done like this:

this.setState({ myArray: [...this.state.myArray, 'new value'] }) //simple value
this.setState({ myArray: [...this.state.myArray, ...[1,2,3] ] }) //another array

Spread syntax

Array push returns length

this.state.myArray.push('new value') returns the length of the extended array, instead of the array itself.Array.prototype.push().

I guess you expect the returned value to be the array.

Immutability

It seems it's rather the behaviour of React:

NEVER mutate this.state directly, as calling setState() afterwards may replace the mutation you made. Treat this.state as if it were immutable.React.Component.

I guess, you would do it like this (not familiar with React):

var joined = this.state.myArray.concat('new value');
this.setState({ myArray: joined })