ES6 declaring variables before or in loop
Where and how should I declare new variables used in loops?
A:
const map = new Map(Object.entries(columns));
let cols;
for (let [key, value] of map)
{
cols = value.split('|');
//...
}
B:
const map = new Map(Object.entries(columns));
for (let [key, value] of map)
{
let cols = value.split('|');
//...
}
C:
const map = new Map(Object.entries(columns));
var cols;
for (let [key, value] of map)
{
cols = value.split('|');
//...
}
Probably A or B since everyone says let is the new var, but is there any difference between A and B?
Edited:
The variable cols will be used only inside for. I was wondering if there are some issues if variable is initialized inside loop (for example 100 times). So I wondered if it should be initialized outside loop. (A or B example)
The purpose is not to get access outside loop, but prevent (for example) 100 initialization variable cols inside loop (because let is used inside loop - case B).
In code snippet A, cols is accessible outside of the for too. As let variables are block-scoped, when used let to define variable inside for , the scope of the variable is for that block only. So, in B, the variable cols will not be accessible outside of the for .
C, is similar to A if cols is defined only once. If col is defined twice in the same scope using let will result in error.
Which one to use depends on the use-case.
cols is needed inside for only, then use let cols = ... cols is needed outside of for too, use let cols; before for and then it can be used after for too in the same enclosing scope. Note that, in this case, cols will be the last value assigned in the loop. The difference between A and B is the scope of the var. In first case you can access cols also outside the loop, in the second you can't.
let a;
for (let i =0; i < 2; i++) {
let b = i;
a = i;
}
console.log('a outside the loop:', a);
console.log('b outside the loop:', b);
console.log('i outside the loop:', i); //If `var` was used instead of `let` then `i` would have had global scope下一篇: ES6在循环之前或循环中声明变量