JavaScript – some notes

Recently we’ve been using JS quite extensively.

Here’s a few things in JS that should be watched for (as they’re not obvious and not well known):

 

Numbers are doubles. This means they’re approximations, so don’t expect an exact result:

var d = 0.1 + 0.2; // result is 0.30000000000000004

 

Loops can have labels, which can be used to break out from those loops:

loopOne: for (var i = 0; i < 10; i++) {
        for (var j = 0; j < 10; j++) {
            if (j == 3)
                break loopOne;
            console.log(i + " " + j);
        }
    }

This will stop after printing “0 2”

 

The ‘for..in’ loop will list properties of a prototype object as well (not unexpected), if you want to test if the property was defined on this specific object – double check with hasOwnProperty() method:   

var parent = {
        name:"Bob",
        gender:"male"
    };

    var child = function () {
        this.age = 20;
    };

    child.prototype = parent;
    var ch = new child();
    for (p in ch) {
        console.log(p + ", is own property: " + ch.hasOwnProperty(p));
    }

will print

age, is own property: true 
name, is own property: false 
gender, is own property: false

 

Setting a parent to null or undefined will have no effect on child, but changing its properties value will:

parent.name = "Bill";
delete parent.gender;
for (p in ch) {
    console.log(p + ", is own property: " + ch.hasOwnProperty(p) + ", value: " +  ch[p]);
}

will print 

age, is own property: true, value: 20 
name, is own property: false, value: Bill

 

  Blocks don’t have their own scope. Variables defined in blocks will be accessible outside:

for (var i = 0; i < 2; i++) {
        var boo = 33;
}
console.log(boo);

will print 33

 

JS has constructor for its types, which generally should be avoided:

var ba = new Boolean(false);
var bb = false, bc = false;
console.log(bb === bc); // true
console.log(ba == bb); // true
console.log(ba === bb); // FALSE! 

 

Be careful when using typeof operator:

console.log(typeof {}); // object
console.log(typeof 2); // number
console.log(typeof "abc"); // string
console.log(typeof null); // OBJECT - this is wrong!
console.log(typeof []); // object - not helpful