In JavaScript and Scope IV - Blocks we showed that blocks on their own do not create a new scope when using the var
keyword. Wouldn't it be nice if there were a way to create the toto
variable inside a block so that it would have its own scope, unlike below?
var toto = 'wants';
{
var toto = 'a puppy';
}
console.log(toto); // logs 'a puppy'
ES6 allows block level scope with the let
keyword when declaring variables.
var toto = 'wants';
{
let toto = 'a puppy';
}
console.log(toto); // logs 'wants'
In the code above, the toto
variable inside the block no longer pollutes the value of the global variable because they are now two different variables even if they have the same name.
We can also fix the for
loop example from the previous article.
var buzz = 3;
for (let buzz = 1; buzz < 10; buzz++) {
if (buzz === 9) {
console.log('for loop buzz:', buzz); // logs 9
}
}
console.log('global scope buzz:', buzz); // now logs 3
And we can also fix the if
statement code. This time we use the const
key word so the value cannot be changed.
var cute = 'not';
if (true) {
const cute = 'cat';
}
console.log(cute); // logs 'not'
As you can see from the examples above, both let
and const
will give you block level scope when you need it. In fact, it's one of the reasons to prefer using let
and const
instead of var
. Variables should have as limited scope as possible to avoid the types of unexpected side effects we saw in the previous article.
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