Using es6 it can be done like this:

this.setState({ myArray: [...this.state.myArray, 'new value'] }) //simple valuethis.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 })

Never recommended to mutate the state directly.

The recommended approach in later React versions is to use an updater function when modifying states to prevent race conditions:

Push string to end of the array

this.setState(prevState => ({  myArray: [...prevState.myArray, "new value"]}))

Push string to beginning of the array

this.setState(prevState => ({  myArray: ["new value", ...prevState.myArray]}))

Push object to end of the array

this.setState(prevState => ({  myArray: [...prevState.myArray, {"name": "object"}]}))

Push object to beginning of the array

this.setState(prevState => ({  myArray: [ {"name": "object"}, ...prevState.myArray]}))