Declaration

function test() {
    var a = b = 100;
}
test();
console.log(b);        // ?

The answer is 100 instead of undefined. Following is what actually happened:

function test() {
    var a = undefined;
    b = 10;
    a = b;
}
...

How about using 'use strict'?

(function() {
    'use strict';
    var a = window.b = 5;
})();
console.log(b);

NaN

1. When is NaN be produced?

// Only '+' sign will try to do some concat.
// Other operations ( / * - ) will produce NaN when number and string are mixed.
console.log('abc'/4);
console.log(4*'a');

// How to check NaN:
Number.isNaN(NaN);    // true, only if is NaN

// Be careful of pitfalls from isNaN() (not Number.isNaN()):
isNaN({});         // true
isNaN('a');        // true
typeof(NaN);       // "number"
NaN === NaN        // false

+ - * /

1. What will be logged?

let a = 10/3;
a === 3;

console.log(2 + '1');            // 21  
console.log('2' + 1);            // 21

console.log(2 / '1');            // 2
console.log(2 - '1');            // 1
console.log('2' - 1);            // 1

console.log('2' - 'a');          // NaN

console.log(-'1');               // -1
console.log(+'-1');              // -1

2. Does it equal? Why?

const n = 0.1 + 0.2;
console.log(n === 0.3);

Check Types

1. How to check if is object?

toString.call({});                      // "[object Object]"
Object.prototype.toString.call(obj);    // "[object Object]"
if (obj instanceof Object) {...}        // true

2. How to check array?

const arr = [];

Array.isArray([]);                      // true, IE 9+
toString.call([]);                      // "[object Array]"
Object.prototype.toString.call(arr);    // "[object Array]"
if (arr instanceof Array) {...}         // true

3. How to check null, undefined, NaN?

val === null;             // true (if it is null)
val === undefined         // true (if it is undefined)
Number.isNaN(NaN)         // true

forEach(), for...in and for...of

What's the difference?

1. forEach is only for Arrays.
2. for...of is only for iterable objects, means cannt use it on objects.
3. for...in is used for loop properties of objects. But it only loops enumerable properties. For example it doesn't touch an array's length property.
4. for...of only work with collections.
5. loops cannot be stopped for all of the 3.

References

1. Check if object is array? http://stackoverflow.com/questions/4775722/check-if-object-is-array
2. “foreach” vs “for of” vs “for in” in JavaScript http://qnimate.com/foreach-vs-for-of-vs-for-in-in-javascript/