Block scope variable – ECMAScript 6

May 3, 2015

Since some of ECMAScript 6 features looks weird to old-fashioned JavaScripters, I’m going to write a series of blog posts to describe the main idea of some common ECMAScript 6 features.

In this posts I’d like to explain the usage of block scope variables. The keyword let enables you to create a block scope variable in ECMAScript 6:

let boo = 1;

So, what’s the difference between var boo = 1; and let boo = 1;? Assume following example:

function sample() {
  if (true) {
    var boo = 1;
  }
  return boo;
}

console.log(sample()); //returns 1

In the above old-fashioned code, boo variable is defined within the sample function scope so we have the value even outside of the if statement.

With let variable you can create a variable that is available within the if statement block:

function sample() {
  if (true) {
    let boo = 1;
  }
  return boo; //ReferenceError: boo is not defined
}

console.log(sample());

Awesome, isn’t it?

I will write more blog posts about ECMAScript 6 features soon.

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Afshin Mehrabani

Block scope variable – ECMAScript 6