let, var and const variables in JavaScript or ReactJS

In ReactJS, the differences between let, var, and const variables are the same as in JavaScript. Here's a brief explanation of each:  

   

1. let: The let keyword declares a block-scoped variable that can be reassigned. It allows you to define variables that are limited in scope to the block, statement, or expression in which they are used. For example, you can use let variables inside loops or conditional statements.  

   Examples:

let lx;

jsvariables(lx)

function jsvariables(lx) {
    
    console.log(lx)
}

console.log(lx);


//Output

undefined
undefined

let lx = 5;

jsvariables(lx)

function jsvariables(lx) {
    lx = 7;
    console.log(lx)
}

console.log(lx);


//Output

7
5

let lx = 5;

jsvariables(lx)

function jsvariables(lx) {
    let lx = 7;
    console.log(lx)
}

console.log(lx);


//Output

let lx = 7;
        ^

SyntaxError: Identifier 'lx' has already been declared

jsvariables()

function jsvariables() {
    let lx = 7;
    console.log(lx)
}

console.log(lx);


//Output

7

console.log(lx);
            ^

ReferenceError: lx is not defined

2. var: The var keyword declares a function-scoped variable that can be reassigned. It is the older way of declaring variables in JavaScript before the introduction of let and const. Unlike let, var declarations are not limited to the block scope but rather to the function scope. This means that var variables are accessible anywhere within the function in which they are declared.  

   Examples:

var vx =12;

jsvariables(vx)

function jsvariables(vx) {
    console.log(vx)
}

console.log(vx);


//Output

12
12

var vx = 12;

jsvariables(vx)

function jsvariables(vx) {
    var vx = 13;
    console.log(vx)
}

console.log(vx);


//Output

13
12

var vx = 12;

jsvariables(vx)

function jsvariables(vx) {
    vx = 13;
    console.log(vx)
}

console.log(vx);


//Output

13
12

var vx;

jsvariables(vx)

function jsvariables(vx) {
    var vy = 12;
    console.log(vy)
}

console.log(vx);


//Output

12
undefined

var vx;

jsvariables(vx)

function jsvariables(vx) {
    vx = 12;
    console.log(vx)
}

console.log(vx);


//Output

12
undefined

var vx;

jsvariables(vx)

function jsvariables(vx) {
    vx = 12;
    console.log(vx)
}
vx = 16;
console.log(vx);


//Output

12
16

3. const: The const keyword declares a block-scoped variable that cannot be reassigned. It is used to declare constants, which are variables that cannot be changed after they are declared. const variables must be initialized with a value at the time of declaration, and you cannot reassign them later.  

   Examples:

const cx = 12;

jsvariables(cx)

function jsvariables(cx) {
    console.log(cx)
}

console.log(cx);


//Output

12
12

const cx;

jsvariables(cx)

function jsvariables(cx) {
    console.log(cx)
}

console.log(cx);


//Output

const cx;
      ^^

SyntaxError: Missing initializer in const declaration

const cx;
cx = 12;
jsvariables(cx)

function jsvariables(cx) {
    
    console.log(cx)
}

console.log(cx);


//Output

const cx;
      ^^

SyntaxError: Missing initializer in const declaration

In general, it is recommended to use const for variables that will not be reassigned, let for variables that will be reassigned within their scope, and avoid using var as it has some potential issues related to scoping and hoisting. However, the choice between let and const ultimately depends on your specific use case and requirements.

TypeError: body used already for url

Error: TypeError: body used already for endpoint url

The reason of this issue is, uses of statement response (e.g. reposne.json()) more than once. If we already use response body statment for reading the response data and try to use again response.json() statement anywhere else or in then function.

Solution: Create a variable and assign the fetch response into that variable as a value to further use. This way we need to call the response.json() statment only once. 

Human Mind is Tabula Rasa | Aristotle

The Human Mind is Tabula Rasa. - Aristotle