JavaScript let: Declaring Block-Scoped Variables
Summary: in this tutorial, you will learn how to use the JavaScript let keyword to declare block-scoped variables.
Introduction to the JavaScript let keyword
In ES5, when you declare a variable using the var keyword, the scope of the variable is either global or local. If you declare a variable outside of a function, the scope of the variable is global. When you declare a variable inside a function, the scope of the variable is local.
ES6 provides a new way of declaring a variable by using the let keyword. The let keyword is similar to the var keyword, except that these variables are blocked-scope. For example:
let variable_name;Code language: JavaScript (javascript)
In JavaScript, blocks are denoted by curly braces {} , for example, the if else, for, do while, while, try catch and so on:
if(condition) {
// inside a block
}Code language: JavaScript (javascript)
See the following example:
let x = 10;
if (x == 10) {
let x = 20;
console.log(x); // 20: reference x inside the block
}
console.log(x); // 10: reference at the begining of the scriptCode language: JavaScript (javascript)
How the script works:
- First, declare a variable
xand initialize its value to 10. - Second, declare a new variable with the same name
xinside theifblock but with an initial value of 20. - Third, output the value of the variable
xinside and after theifblock.
Because the let keyword declares a block-scoped variable, the x variable inside the if block is a new variable and it shadows the x variable declared at the top of the script. Therefore, the value of x in the console is 20.
When the JavaScript engine completes executing the if block, the x variable inside the if block is out of scope. Therefore, the value of the x variable that following the if block is 10.
JavaScript let and global object
When you declare a global variable using the var keyword, you add that variable to the property list of the global object. In the case of the web browser, the global object is the window. For example:
var a = 10;
console.log(window.a); // 10Code language: JavaScript (javascript)
However, when you use the let keyword to declare a variable, that variable is not attached to the global object as a property. For example:
let b = 20;
console.log(window.b); // undefinedCode language: JavaScript (javascript)
JavaScript let and callback function in a for loop
See the following example.
for (var i = 0; i < 5; i++) {
setTimeout(function () {
console.log(i);
}, 1000);
}Code language: JavaScript (javascript)
The intention of the code is to output numbers from 0 to 4 to the console every second. However, it outputs the number 5 five times:
5
5
5
5
5
In this example, the variable i is a global variable. After the loop, its value is 5. When the callback functions are passed to the setTimeout() function executes, they reference the same variable i with the value 5.
In ES5, you can fix this issue by creating another scope so that each callback function references a new variable. And to create a new scope, you need to create a function. Typically, you use the IIFE pattern as follows:
for (var i = 0; i < 5; i++) {
(function (j) {
setTimeout(function () {
console.log(j);
}, 1000);
})(i);
}Code language: JavaScript (javascript)
Output:
0
1
2
3
4
In ES6, the let keyword declares a new variable in each loop iteration. Therefore, you just need to replace the var keyword with the let keyword to fix the issue:
for (let i = 0; i < 5; i++) {
setTimeout(function () {
console.log(i);
}, 1000);
}Code language: JavaScript (javascript)
To make the code completely ES6 style, you can use an arrow function as follows:
for (let i = 0; i < 5; i++) {
setTimeout(() => console.log(i), 1000);
}Code language: JavaScript (javascript)
Note that you’ll learn more about the arrow functions in the later tutorial.
Redeclaration
The var keyword allows you to redeclare a variable without any issue:
var counter = 0;
var counter;
console.log(counter); // 0Code language: JavaScript (javascript)
However, redeclaring a variable using the let keyword will result in an error:
let counter = 0;
let counter;
console.log(counter);Code language: JavaScript (javascript)
Here’s the error message:
Uncaught SyntaxError: Identifier 'counter' has already been declaredCode language: JavaScript (javascript)
JavaScript let variables and hoisting
Let’s examine the following example:
{
console.log(counter); //
let counter = 10;
}Code language: JavaScript (javascript)
This code causes an error:
Uncaught ReferenceError: Cannot access 'counter' before initializationCode language: JavaScript (javascript)
In this example, accessing the counter variable before declaring it causes a ReferenceError. You may think that a variable declaration using the let keyword does not hoist, but it does.
In fact, the JavaScript engine will hoist a variable declared by the let keyword to the top of the block. However, the JavaScript engine does not initialize the variable. Therefore, when you reference an uninitialized variable, you’ll get a ReferenceError.
Temporal death zone (TDZ)
A variable declared by the let keyword has a so-called temporal dead zone (TDZ). The TDZ is the time from the start of the block until the variable declaration is processed.
The following example illustrates that the temporal dead zone is time-based, not location-based.
{ // enter new scope, TDZ starts
let log = function () {
console.log(message); // messagedeclared later
}; // This is the TDZ and accessing log
// would cause a ReferenceError
let message= 'Hello'; // TDZ ends
log(); // called outside TDZ
}
Code language: JavaScript (javascript)
In this example:
First, the curly brace starts a new block scope, therefore, the TDZ starts.
Second, the log() function expression accesses the message variable. However, the log() function has not been executed yet.
Third, declare the message variable and initialize its value to 'Hello'. The time from the start of the block scope to the time that the message variable is accessed is called a temporal death zone. When the JavaScript engine processes the declaration, the TDZ ends.
Finally, call the log() function that accesses the message variable outside of the TDZ.
Note that if you access a variable declared by the let keyword in the TDZ, you’ll get a ReferenceError as illustrated in the following example.
{ // TDZ starts
console.log(typeof myVar); // undefined
console.log(typeof message); // ReferenceError
let message; // TDZ ends
}Code language: JavaScript (javascript)
Notice that myVar variable is a non-existing variable, therefore, its type is undefined.
The temporal death zone prevents you from accidentally referencing a variable before its declaration.
Summary
- Variables are declared using the
letkeyword are block-scoped, are not initialized to any value, and are not attached to the global object. - Redeclaring a variable using the
letkeyword will cause an error. - A temporal dead zone of a variable declared using the
letkeyword starts from the block until the initialization is evaluated.