CODEWITHSUNDEEP
javascriptreactjs
FranktheTank
FranktheTank

Reputation: 292

Where do function parameters come from in javascript?

I am using the following code:

  handleOwnerMode = ownerChecked  => {
     this.setState(prev => ({ ownerChecked, showOwner: !prev.showOwner}))
     // this.setState(prev => ({ ownerChecked: !prev.ownerChecked, showOwner: !prev.showOwner }))
  }

Inside the render is 

<Switch onChange={this.handleOwnerMode} checked={this.state.ownerChecked} />

OnChange, I somehow was able to receive what was changed about ownerChecked. Why is this the case? I didn't write onChange={this.handleOwnerMode(event.value)} or anything like that...

Also, for setState. I normally just use this.setState( { state: newState} ), but now I can somehow pass in the previous state with this.setState( prev => {} ). Is there defined overloading somewhere that lets me do this?

Thank you.

Upvotes: 0

Views: 85

Answers (2)

Yusuf Altıparmak
Yusuf Altıparmak

Reputation: 466

React events are synthetic so that even when yo do not pass any event parameters, function takes it. Take a look at these. More information 1, More information 2.

Upvotes: 1

Giorgi Moniava
Giorgi Moniava

Reputation: 28654

OnChange, I somehow was able to receive what was changed about ownerChecked. Why is this the case? I didn't write onChange={this.handleOwnerMode(event.value)} or anything like that...

In both cases you have passed a function (callback) to "receivers". First time to Switch Component, second time to React. Each of them can call your function/callback using any parameter they want. This is how normally callbacks are used.

Is there defined overloading somewhere that lets me do this?

Yeah probably setState checks if you passed a function to it and behaves differently: e.g. gives you previous state and calls your function. If you pass an object to it, it doesn't do that.

Upvotes: 3

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