Console Learner — An interactive JavaScript course.

Declaring variables inside a function used to be the only way to avoid creating global variables. It has therefore been very common to wrap code in an anonymous function to control global namespace export.

Since let and const was introduced (in ES6), it is now also possible to bind variables to a statement block.

The image below shows the scopes used in this example:

JavaScript unfortunately lacks a mechanism with which to introspect name bindings in a scope (except for the global scope).

The possibilities and implications of function closures are sometimes hard to understand, but we'll revisit the topic later. In particular this has repercussions on memory usage, as a reachable scope (and its bindings) cannot be reclaimed.

var a = 1;

function test() {
    var a = 2;
    return a;
}

test()

2

a

1

function inc() {
    return b++;
}

inc()

ReferenceError: Can't find variable: b

var b = 13;

inc()

13

The console allows you to interact with the course material and examples. Use the following keys:

• <Enter> — Execute the code
• <Shift + Enter> — Add line break
• <Arrow-Up> — Previous command in history
• <Arrow-Down> — Next command in history

A special logging function is also available:

• print(value) — Prints a value to the console.
• print(value, true) — Prints a debug version of a value.