How react functional component can use object variable which declared behind the usage? (include example)

user_11650728 0
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import React from "react";
import "./styles.css";

const App = () => {
  return (
    <div className="App">
      <h1>{test.foo}</h1>
    </div>
  );
};

const test = {
  foo: "foo"
};

export default App;

this code is from codesandbox : https://codesandbox.io/s/declaring-variable-test-oixgo

this code run without any issue, I thought in javascript, const and let are not hoisted,
so It should be declared and initialized first before It is used (I thought the order of executing code is important)
But In this case, Although test object is behind its usage, This code works without any error.

What has happened to this code?

reactjs javascript
3 Answers

Functions don't try to access variables until the function is called.

The function isn't called before the const statement.

Dorward_190680

Check this example:

const App = () => {
  console.log("read",test.foo)
};

App()

const test = {
  foo: "foo"
};

In React, all code will be read then execute component:

const App = () => {
  console.log("read",test.foo)
};

const test = {
  foo: "foo"
};

App()

user_126087140

const and let are not hoisted

That's not correct. They are hoisted, it's just that they are hoisted differently as compared to how var does.

From the Ecmascript spec - 13.3.1 Let and Const Declarations

let and const declarations define variables that are scoped to the running execution context's LexicalEnvironment. The variables are created when their containing Environment Record is instantiated but may not be accessed in any way until the variable's LexicalBinding is evaluated. A variable defined by a LexicalBinding with an Initializer is assigned the value of its Initializer's AssignmentExpression when the LexicalBinding is evaluated, not when the variable is created.

Reason why your code works because App function is called after test object's declaration has already been processed.

To understand this better, consider the following example:

const foo = () => {
  console.log(test.a);
};

const test = { a: "Hello" };

foo();

Above code example logs "Hello" because by the time foo function is called, test object's declarations has already been processed.

Now, if we move the foo function call above the test object declaration, you will get an error.

const foo = () => {
  console.log(test.a);
};


foo();

const test = { a: "Hello" };

Above code example throws an error because of Temporal Dead Zone, foo function can't access the test object. foo function can only access test object if it is called after the declaration of test object has been executed during step-by-step execution of the code, which is what happened in the first code example.

user_60943480
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