When I inspect my React code in the Chrome developer tools 'Source' tab, and I hover over the 'this.props' or even 'this' keyword / add it to watches, it shows up as undefined. Even though the code referred to executes successfully. Very annoying.... is this is bug? Is there a workaround for this?
If the expression doesn't return a value, the console will report undefined — this doesn't mean anything is wrong, it's just letting you know that the previous expression didn't return a result.
The quickest way to open React Devtools is to right click your page and select inspect. If you've used Chrome or Firefox's developer tools, this view should look a little familiar to you.
Developer tools (or "development tools" or short "DevTools") are programs that allow a developer to create, test and debug software. Current browsers provide integrated developer tools, which allow to inspect a website.
It's undefined
because you're inside an arrow function and by definition an arrow function doesn't own a context but it inherits the enclosing one.
If you check the menu on the right handside and scroll down to the current scope you will find the closure chain of that function (one of which in your case will be the component class) that has the this
inherited by your function.
e.g.
An arrow function does not have its own this. The this value of the enclosing lexical scope is used;
Arrow functions doc
Due to how lexical this
is treated by Babel in arrow functions, it cannot be this
inside an arrow. Temporary variables like _this
, _this2
, etc. are used to imitate lexical this
in ES5.
Transpiled code looks like:
var _this = this;
...
.then(function () {
...
_this.setState(...);
});
Even though it appears like original ES6 source in debugger because of sourcemaps, it's ES5 that is evaluated. So it's _this
local variable that needs to be debugged.
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