6 things I didn’t know you can do in Javascript

DanielJuly 8, 2020July 8, 2020

The fact that JavaScript is such a flexible language makes it maybe the easiest language to start with and the hardest to master. I’ve been working with JavaScript for more than 10 years now and I still stumble upon some hidden syntax or tricks that I never knew existed.

So let's see some JS mysteries that I've recently discovered.

1. There is a function constructor. You will give it the parameters and the body of the function and it will return back the actual function :

const diff = new Function('a', 'b', 'return a - b');
diff(20,13) // 7

2. We have a with statement in Javascript. It takes as a parameter an object and binds the properties of that object to the scope of the contained code block.

const book = {
    author: 'Ernest Hemingway',
    title: 'The Sun Also Rises'
}
with(book) {
    console.log(author); // Ernest Hemingway
    console.log(title); // The Sun Also Rises
}

3. we can use the + operator to convert a string to a number. No more the need to use functions like parseInt() or parseFloat(), unless you want to parse to that specific numerical type.

const nr = +'1.5';
nr + 1; // 2.5

4. we can assign properties to functions. We can make configurable functions by assigning specific properties to that function.

function sayHello() {
    if (sayHello.country = 'US') {
        return alert('Hi there!');
    }
    if (sayHello.country = 'FR') {
        return alert('Bonjour !');
    }
    if (sayHello.country = 'GR') {
            return alert('Guten Tag !');
        }
    return alert('Hi');
}
sayHello.country =  'FR';
sayHello(); // alert('Bonjour !');

Also, we can use these function properties as counters or "static variables".

5. we can use the arguments.callee.caller to see what function invoked the current function. The arguments var is a default of any js function. But we have the arguments.callee.caller that will tell us who called that function. Something like a console.trace() but just one level deep.

function sayHello() {
    alert(arguments.callee.caller); //start
}

function start() {
    sayHello();
}()

Also the arguments.callee refers to the currently running function.

6. we have a void operator. You give it anything and it returns back undefined.

void(1); // undefined
void(true); // undefined
void(false); // undefined
void({}); // undefined

You may ask yourself why you would want an operator like this one? Well because before ES5 you could actually assign a new value to the original undefined:

undefined = "abc";
x = undefined;
alert(x); // "abc"
// this will not work anymore
// but it explains why we have the void operator 

So ... Javascript the language where surprises never end!