Avoid Creating Variables Using var

console.log(varVariable) // undefined
var varVariable = 10;
console.log(varVariable) // 10

Variables created with var are hoisted meaning the declaration is moved to the top of their scope before code execution. In simple terms, we can refer to the variable before it's declared. Since the variable is not yet initialized, it has an undefined value.

What about const and let?

console.log(letVariable) // ReferenceError
let letVariable = 10;

console.log(constVariable) // ReferenceError
const constVariable = 10;

Referencing the value of a variable declared using const and let before its declaration will result in ReferenceError.

Type Coercion i.e.Implicit Type conversion

Javascript performs type coercion when an operation has mismatched types. instead of throwing type errors.

It is very convenient, but can also result in unexpected bugs if the developer didn't intend for that conversion. Here are a few examples.

"50" + 1 // "501"
"50" - 1 // 49
true + true + true // 3
false - true // -1
[] + [] // ""
{} + [] // 0
[] + {} // "[object Object]"
"foo" + + "bar" // "fooNaN"
Number.MIN_VALUE > 0 // true
null == false // false
"222" - -"111" // 333
3 > 2 > 1 // false
true == "true" // true
false == "false" // false
[1, 2, 3] + [4, 5, 6] // "1,2,34,5,6"
!+[]+[]+![] // "truefalse"

1. "50" + 1 results in "501". The number 1 is coerced to a string and concatenates with the other string. Hence the final result is "501".

2. "50" - 1 results in 49. The string "50" is coerced to a number and performs subtraction. Hence the final result is 49.

3. true + true + true results in 3. true when coerced to number returns 1. Therefore 1 + 1 + 1 results in 3.

4. false - true results in -1. Similar to true, false when coerced to number returns 0. Therefore 0 - 1 results -1.

5. [] + [] results in "". [] is coerced to an empty string. Therefore "" + "" results "".

6. {} + [] results in 0. {} is an empty block. and + [] is coerces [] to a number which is equal to 0. Hence the final result is 0.

7. [] + {} results in "". [] is coerced to an empty string. {} is an object and when it is concatenated with an empty string will result in "[object Object]".

8. "foo" + + "bar" results in "fooNaN". "foo" is a string. + "bar" convert the string "bar" to a number which will return NaN. "foo" + NaN will return "fooNaN".

9. Number.MIN_VALUE > 0 results true. Number.MIN_VALUE returns 5e-324 which is the smallest positive number that can be represented within float precision, i.e. that's as close as you can get to zero.

10. null == false results in false. Because null is only comparable with null or undefined to return true, otherwise it will return false.

11. "222" - -"111" results in 333. - "111" is converted to a number and return -111. then "222" in "222" - -111 is coerced to a number 222. Finally 222 - -111 results in 333.

12. 3 > 2 > 1 results in false. First 3 > 2 is evaluated which results in true. Next true > 1 in this case true is converted to a number 1 > 1 which returns false.

13. true == "true" return true since both true and "true" is truthy.
    false == "false" returns false, because false is falsy but "false" is truthy.

14. [1, 2, 3] + [4, 5, 6] results in "1,2,34,5,6". Both the arrays are first converted into strings which look like "1,2,3" + "4,5,6" which results in "1,2,34,5,6".

15. !+[]+[]+![] results in "truefalse". This is how the expression is evaluated
    +[] is converted into a number which results in 0,
    ![], [] is a truth, therefore !true is false,
    []+![] becomes []+false, where [] is covered to "" . Therefore ""+false results in "false",
    !+[] becomes !false, which results in true.
    Finally, !+[]+[]+![] becomes true+"false" which results in "truefalse"

typeof null === "object"

In the first implementation of JavaScript, JavaScript values were represented as a type tag and a value. The type tag for objects was 0. null was represented as the NULL pointer (0x00 in most platforms). Consequently, null had 0 as type tag, hence the typeof return value "object".

From MDN documentation refer.

x !== x can be true

When x = NaN, NaN is the only value that compares unequal to itself. Hence if you need to check for NaN condition, we have to use Number.isNaN() or isNaN().

The reason why NaN is unequal to itself. It's possible to produce two floating point numbers with different binary representations but are both NaN.

Mutable Global variables

var console = {}
console.log() // Uncaught TypeError: console.log is not a function

const example = () => {
var undefined = 10;
console.log(undefined) // 10
}
example()

undefined is a property of the global object. It is mutable in any scope other than global scope.

Modifying the global variable is "A good way to make your life more difficult."

Automatic Semicolon Insertion

// How to intend the code to execute.
const getUser = () => {
    return
    { name: 'John' };
}
console.log(getUser()); // { name: 'John' }

This is what Javascript sees, JS automatically adds semi-colon, if the return statement is followed by a new line terminator.

// How JS executes the code.
const getUser = () => {
    return;
    { name: 'John' }; // Unreachable code.
}
console.log(getUser()); // undefined

Arrays

const arr = [];
console.log(arr.length); // 0
arr.length = 50;
console.log(arr.length); // 50
arr.length = 30;
console.log(arr.length); // 30

Array syncs the contents of the array automatically when the length value changes. It can be used to increase or decrease the contents of an array. The value of length should always be a positive integer value.

Min and Max Comparison

Math.min() < Math.max() // false
Math.max() // -Infinity
Math.min() // Infinity

Math.min() to find the lowest number within the parameter, will start the comparison with the largest positive integer which is Infinity. Since there are no parameters to be compared with, it returns Infinity.

Similarly, Math.max() to find the highest number within the parameters, will start the comparison with the smallest negative integer which is -Infinity. Since there are no parameters to be compared with, it returns -Infinity.

Numerical Array Sort

const arr = [10,1,3,40,8,9,23]
arr.sort()
console.log(arr) // [1, 10, 23, 3, 40, 8, 9]

Array.sort() converts the number to a string and then compares strings in UTF-16 code units order. In that case "10" comes before "9".