Some useful JS techniques that you may not know

I complete reading JavaScript Enlightenment recently. The book is more about basics in JavaScript and suitable for beginners. Here are a list of my benefits and extensions from the book.

Math

JavaScript developers often capitalize the constructor name to distinguish the constructors from normal functions. Therefore, some developers may mistake Math as function since the name is capitalized while Math is really just an object.

console.log(typeof Math); // object
console.log(typeof Array); // function

Function

There are two ways to construct a function:

• Function declaration/expression

function sum(x, y) {
    return x + y;
}
console.log(sum(3,4)); //7

• Function constructor

let multiply = new Function("x", "y", "return x * y;");
console.log(multiply(3,4)); //12

In development, we often need to use call or apply to switch the function context. E.g.

function print(){
    console.log(this.a);
}

print.call({a: 'hello'}); //hello

Some interview questions will ask why print doesn't define call as its property but print.call() won't throw error. It's because print is an instance from Function constructor so it inherits all the methods defined in Function.prototype through prototype chain.

print.call === Function.prototype.call

How to do Array check

typeof can be used to determine the types for primitive datatypes. But it won't be able to distinguish between arrays and objects.

console.log(typeof [1,2]); //object
console.log(typeof {}); //object

There are several wasy to do Array check:

• API: Array.isArray

console.log(Array.isArray([1,2])); //true

• toString

console.log(Object.prototype.toString.call([1,2])
            .toLowerCase() === '[object array]'); //true

Note here we have to use Object.prototype.toString with call to switch the calling context, as Array.prototype.toString is overriden.

console.log(Object.prototype.toString.call([1,2])); //[object Array]
console.log([1,2].toString()); //1,2

• instanceof

[1,2] instanceof Array === true;

Object.prototype.toString won't work for custom datatypes. In this case we can use instanceof to determine the type.

class Person {}

let mike = new Person();
console.log(Object.prototype.toString.call(mike)); // [object Object]
console.log(mike instanceof Person); // true

undefined vs null

undefined - no value

There are two cases where a variable is undefined.

• The variable is deifned but not assigned any value.

let s;
console.log(s); //undefined

• The variable is NOT defined at all.

let obj1 = {};
console.log(obj1.a); //undefined

null - special value

let obj2 = {a: null};
console.log(obj2.a); //null

If we aim to filter out undefined and null, we can use weak comparison with double equality sign(i.e. ==).

console.log(null == undefined); //true
let arr = [null, undefined, 1];
let fil = arr.filter(it => {
    return it != null;
});
console.log(fil); //[1]

Reference

JavaScript Enlightenment

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