ES2015(aka ES6) introduced a new keyword let that declares a new variable in block scope. Before that, most JavaScript variables are function scope or global scope.

Function Scope

Following example shows the scope of variable num declared by var.

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(function () {
  {
    var num = 1;
    console.log(num);  // 1
  }
  console.log(num);  // 1. Same variable as defined in {}.
}());

Block Scope

Following example shows the scope of variable num declared by let.

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(function () {
  {
    let num = 1;
    console.log(num);  // 1
  }  // num goes out of scope.
  console.log(num);  // ReferenceError: num is not defined.
}());

Hoisting

Variables declared by var are hoisted.

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(function () {
  console.log(num);  // undefined
  {
    var num = 1;
    console.log(num);  // 1
  }
}());

Variables declared by let are not hoisted.

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(function () {
  console.log(num);  // ReferenceError: num is not defined.
  {
    let num = 1;
    console.log(num);  // 1
  }
}());

Block Scope Before ES2015

Variables declared in the catch clause of try...catch statement is block scoped.

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