Arrow function should not return assignment?
My code is working correctly in the app however, my eslint isn't liking it and is saying I should not return assignment. What is wrong with this?
<div ref={(el) => this.myCustomEl = el} />
The Fix:
<div ref={(el) => { this.myCustomEl = el }} />
The Explanation:
Your current code is equivalent to:
<div ref={(el) => { return this.myCustomEl = el }} />
You are returning the result of this.myCustomEl = el. In your code, this is not really a problem -- however, one of the most frustrating bugs in programming occurs when you accidentally use an assignment (=) instead of a comparator (== or ===), for instance:
// This function will always return **true**, surprisingly
function isEqual(a, b) {
// The warning would be thrown here, because you probably meant to type "a===b". The below function will always return true;
return a=b;
}
let k=false;
let j=true;
if(isEqual(k,j)){
// You'll be very, very, very confused as to why this code is reached because you literally just set k to be false and j to be true, so they should be different, right? Right?
thisWillExecuteUnexpectedly();
}
In the above case, the compiler warning makes sense because k=true
evaluates to true (as opposed to k===true
, which is probably what you meant to type) and causes unintended behavior. Thus, eshint notices when you return an assignment, assumes that you meant to return a comparison, and lets you know that you should be careful.
In your case, you can solve this by simply not returning the result, which is done by adding enclosing brackets {} and no return statement:
<div ref={(el) => { this.myCustomEl = el }} />
You can also adjust the eshint warning like so: https://eslint.org/docs/rules/no-return-assign
You're implicitly returning an assignment. this.myCustomEl = el
is an assignment. You could fix this linting error by changing your arrow function to (el) => { this.myCustomEl =el }
which is no longer implicitly returning because you wrapped it in {}
instead of ()
.
Side note: Declaring an arrow function inline inside a render method will break a PureComponent
because every time your component renders it has to declare a new anonymous function, so the shallow props comparison that a PureComponent
does is broken by this and will always re-render.
Try making that a method of your component.
class MyClass extends React.PureComponent {
getRef = (el) => { this.ref = el; }
render() {
return <div ref={this.getRef} />;
}
}
If the above syntax doesn't work for you, you can use the following:
class MyClass extends React.PureComponent {
constructor(props) {
super(props);
this.ref = null;
this.getRef = this.getRef.bind(this);
}
getRef(el) {
this.ref = el;
}
render() {
return <div ref={this.getRef} />;
}
}