How can I assign an array to a state variable?

Question

I have a function that looks like this:

select(e) {
    let { selectedItems } = this.state;

    let arr: number[] = [];
    for (var i = 0; i < 2; i++) arr = [...arr, i];

    this.setState(
      {
        selectedItems: arr,
      }
    );
  console.log(selectedItems);
  }

But when I try to console.log(selectedItems), all I get is an empty array. When I console.log(arr), I get the correct output I'm looking for -- an array of length 2. What might be going on here and how can I assign selectedItems to the value of arr?

Where/when do you log? (I don't understand your sample code.) `setState` is async. Being an array isn't relevant; `setState` doesn't care about types. — Dave Newton, Mar 30 '20 at 17:30
https://stackoverflow.com/questions/36085726/why-is-setstate-in-reactjs-async-instead-of-sync — epascarello, Mar 30 '20 at 17:33

score 1 · Answer 1 · edited Mar 30 '20 at 19:57

1

setState takes a callback function. Try the following:

  select(e) {
    let { selectedItems } = this.state;

    let arr: number[] = [];
    for (var i = 0; i < 2; i++) arr = [...arr, i];

    this.setState({
      selectedItems: arr,
    }, function(){
        console.log(this.state);
    }.bind(this));
  }

edited Mar 30 '20 at 19:57

Edgar Henriquez

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answered Mar 30 '20 at 17:44

DCR

14,737
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This won't work, you need to bind the `this` keyword to that callback function. – Edgar Henriquez Mar 30 '20 at 17:46
check the answer below. not binded and appears to work – DCR Mar 30 '20 at 18:00

score 1 · Accepted Answer · answered Mar 30 '20 at 17:51

1

class A {
  select(e) {
    let { selectedItems } = this.state;
    let arr: number[] = [];
    for (var i = 0; i < 2; i++) arr = [...arr, i];
    this.setState({
      selectedItems: arr
    }, function callback() {
      console.log(this.state) // will have updated value
    });
    console.log(selectedItems); // wont print anything, coz u never assign
    selectedItems = arr;
    console.log(selectedItems); // will have updated value
  }
}

answered Mar 30 '20 at 17:51

xdeepakv

7,835
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thanks, how silly of me, I just realized that this is what I was missing all along – helloWorld Mar 30 '20 at 17:59

How can I assign an array to a state variable?

2 Answers2