Javascript – Prop destructuring inside of react state

eslintjavascriptreactjs

I've added airbnb config for eslint that encourages prop and state destructuring, I like it, but stumbled across one problem when I define state in my component

class MyComponent extends Component {
  state = {
    animation: this.props.active ? 1 : 0
  }

I get an error

[eslint] Must use destructuring props assignment
(react/destructuring-assignment)

I'm not sure how can I correctly destructure active out of props here?
Usually const {active} = this.props works, but whenever I place it inside state or around it I get unexpected syntax error.

Best Answer

The only to keep it inside of the class property is to use a getter (which will be invoked at the first render):

state = {
  get animation() {
    const { active } = this.props;
    return active ? 1 : 0;
  }
}

Or you use an IIFE to initialize the property:

state = (() => {
  const { active } = this.props;
  return { animation: active ? 1 : 0 };

})()

But thats actually a bit overcomplicating. Another solution would be to move the property into the constructor:

constructor(...args) {
 super(...args);

 const { active } = this.props;
 this.state = { animation: active ? 1 : 0 };

}

But I personally would just ignore that warning here.

ES6 solution: `let`

ECMAScript 6 (ES6) introduces new let and const keywords that are scoped differently than var-based variables. For example, in a loop with a let-based index, each iteration through the loop will have a new variable i with loop scope, so your code would work as you expect. There are many resources, but I'd recommend 2ality's block-scoping post as a great source of information.

for (let i = 0; i < 3; i++) {
  funcs[i] = function() {
    console.log("My value: " + i);
  };
}

Beware, though, that IE9-IE11 and Edge prior to Edge 14 support let but get the above wrong (they don't create a new i each time, so all the functions above would log 3 like they would if we used var). Edge 14 finally gets it right.

ES5.1 solution: forEach

With the relatively widespread availability of the Array.prototype.forEach function (in 2015), it's worth noting that in those situations involving iteration primarily over an array of values, .forEach() provides a clean, natural way to get a distinct closure for every iteration. That is, assuming you've got some sort of array containing values (DOM references, objects, whatever), and the problem arises of setting up callbacks specific to each element, you can do this:

var someArray = [ /* whatever */ ];
// ...
someArray.forEach(function(arrayElement) {
  // ... code code code for this one element
  someAsynchronousFunction(arrayElement, function() {
    arrayElement.doSomething();
  });
});

The idea is that each invocation of the callback function used with the .forEach loop will be its own closure. The parameter passed in to that handler is the array element specific to that particular step of the iteration. If it's used in an asynchronous callback, it won't collide with any of the other callbacks established at other steps of the iteration.

If you happen to be working in jQuery, the $.each() function gives you a similar capability.

Classic solution: Closures

What you want to do is bind the variable within each function to a separate, unchanging value outside of the function:

var funcs = [];

function createfunc(i) {
  return function() {
    console.log("My value: " + i);
  };
}

for (var i = 0; i < 3; i++) {
  funcs[i] = createfunc(i);
}

for (var j = 0; j < 3; j++) {
  // and now let's run each one to see
  funcs[j]();
}

Since there is no block scope in JavaScript - only function scope - by wrapping the function creation in a new function, you ensure that the value of "i" remains as you intended.

Javascript – .prop() vs .attr()

Update 1 November 2012

My original answer applies specifically to jQuery 1.6. My advice remains the same but jQuery 1.6.1 changed things slightly: in the face of the predicted pile of broken websites, the jQuery team reverted attr() to something close to (but not exactly the same as) its old behaviour for Boolean attributes. John Resig also blogged about it. I can see the difficulty they were in but still disagree with his recommendation to prefer attr().

Original answer

If you've only ever used jQuery and not the DOM directly, this could be a confusing change, although it is definitely an improvement conceptually. Not so good for the bazillions of sites using jQuery that will break as a result of this change though.

I'll summarize the main issues:

You usually want prop() rather than attr().
In the majority of cases, prop() does what attr() used to do. Replacing calls to attr() with prop() in your code will generally work.
Properties are generally simpler to deal with than attributes. An attribute value may only be a string whereas a property can be of any type. For example, the checked property is a Boolean, the style property is an object with individual properties for each style, the size property is a number.
Where both a property and an attribute with the same name exists, usually updating one will update the other, but this is not the case for certain attributes of inputs, such as value and checked: for these attributes, the property always represents the current state while the attribute (except in old versions of IE) corresponds to the default value/checkedness of the input (reflected in the defaultValue / defaultChecked property).
This change removes some of the layer of magic jQuery stuck in front of attributes and properties, meaning jQuery developers will have to learn a bit about the difference between properties and attributes. This is a good thing.

If you're a jQuery developer and are confused by this whole business about properties and attributes, you need to take a step back and learn a little about it, since jQuery is no longer trying so hard to shield you from this stuff. For the authoritative but somewhat dry word on the subject, there's the specs: DOM4, HTML DOM, DOM Level 2, DOM Level 3. Mozilla's DOM documentation is valid for most modern browsers and is easier to read than the specs, so you may find their DOM reference helpful. There's a section on element properties.

As an example of how properties are simpler to deal with than attributes, consider a checkbox that is initially checked. Here are two possible pieces of valid HTML to do this:

<input id="cb" type="checkbox" checked>
<input id="cb" type="checkbox" checked="checked">

So, how do you find out if the checkbox is checked with jQuery? Look on Stack Overflow and you'll commonly find the following suggestions:

if ( $("#cb").attr("checked") === true ) {...}
if ( $("#cb").attr("checked") == "checked" ) {...}
if ( $("#cb").is(":checked") ) {...}

This is actually the simplest thing in the world to do with the checked Boolean property, which has existed and worked flawlessly in every major scriptable browser since 1995:

if (document.getElementById("cb").checked) {...}

The property also makes checking or unchecking the checkbox trivial:

document.getElementById("cb").checked = false

In jQuery 1.6, this unambiguously becomes

$("#cb").prop("checked", false)

The idea of using the checked attribute for scripting a checkbox is unhelpful and unnecessary. The property is what you need.

It's not obvious what the correct way to check or uncheck the checkbox is using the checked attribute
The attribute value reflects the default rather than the current visible state (except in some older versions of IE, thus making things still harder). The attribute tells you nothing about the whether the checkbox on the page is checked. See http://jsfiddle.net/VktA6/49/.

Best Answer

Related Solutions

JavaScript closure inside loops – simple practical example

ES6 solution: let

ES5.1 solution: forEach

Classic solution: Closures

Javascript – .prop() vs .attr()

Related Topic

ES6 solution: `let`