什么时候用 let 什么时候还应该用 var

Question

I read an article that we don't need to use var keyword anymore but I ran into a problem in my code where I could not use my variable inside an if block because I had declared it using let.

How could I solve this problem if we don't need to use var?

let elementWidth = el.getBoundingClientRect().width
let windowWidth = window.width // 400 px

if (windowWidth > elementWidth) {
    elementWidth += 200
}

console.log(elementWidth) // 400 px

Answer 1

"let" variable will not change the values outside of block scope as if, for statement

for( let j = 0; j < 3; j++ ) {
    console.log(i);// i = 3
};
console.log(j); //undefined

"var" is for function scope, a global scope like

    for( var i= 0; i < 3; i++ ) {
    console.log(i);
};
console.log(i);// i = 3

for more https://edgecoders.com/function-scopes-and-block-scopes-in-javascript-25bbd7f293d7