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Todd Chaffee
Todd Chaffee

Posted on • Originally published at blog.toddbiz.com

JavaScript and Scope V - let and const

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'

Edit in JSFiddle

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

Edit in JSFiddle

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'

Edit in JSFiddle

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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