The main difference is scoping rules wherein the variables declared by var keyword are scoped to the immediate function body while let variables are scoped to the immediate enclosing block denoted by { }.
function run() {
var foo = "Foo";
let bar = "Bar";
console.log(foo, bar); // Foo Bar
{
var moo = "Mooo"
let baz = "Bazz";
console.log(moo, baz); // Mooo Bazz
}
console.log(moo); // Mooo
console.log(baz); // ReferenceError
}
run();
The reason why the let keyword was introduced to the language was function scope is confusing and was one of the main sources of bugs in JavaScript.
var funcs = [];
// let's create 3 functions
for (var i = 0; i < 3; i++) {
// and store them in funcs
funcs[i] = function() {
// each should log its value.
console.log("My value: " + i);
};
}
for (var j = 0; j < 3; j++) {
// and now let's run each one to see
funcs[j]();
}
My value: 3 was output to console each time funcs[j](); was invoked since anonymous functions were bound to the same variable.
People had to create immediately invoked functions to capture correct values from the loops but that was also hairy.
While variables declared with var keyword are hoisted which means they are accessible in their enclosing scope even before they are declared:
function run() {
console.log(foo); // undefined
var foo = "Foo";
console.log(foo); // Foo
}
run();
Do let variables are not initialized until their definition is evaluated. Accessing them before the initialization results in a ReferenceError. The variable is said to be in "temporal dead zone" from the start of the block until the initialization is processed.
function checkHoisting() {
console.log(foo); // ReferenceError
let foo = "Foo";
console.log(foo); // Foo
}
checkHoisting();
At the top level, let, unlike var, does not create a property on the global object:
var foo = "Foo"; // globally scoped
let bar = "Bar"; // not allowed to be globally scoped
console.log(window.foo); // Foo
console.log(window.bar); // undefined
Finally, In strict mode, var will let you re-declare the same variable in the same scope while let raises a SyntaxError.
'use strict';
var foo = "foo1";
var foo = "foo2"; // No problem, 'foo1' is replaced with 'foo2'.
let bar = "bar1";
let bar = "bar2"; // SyntaxError: Identifier 'bar' has already been declared
