JS

Null and Undefined in javascript

1.Null and Undefined

In javascript an undefined variable is a variable that never been declared or never assigned a value.

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var foo;
console.log(foo); // The output will be: undefined

If you assign foo = null. Then foo will be null which is a javascript object

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var foo;
console.log(typeof foo); //output: undefined

foo = null;
console.log(typeof foo); //output: object

2.Confuse of working with “==” and “===”.

=== means strictly equal which will check both type and value.

Servral samples:

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null === null            # => true
undefined === undefined  # => true
undefined === null       # => false
undefined == null        # => true

3.What is false in javascript?

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if(!false) {
    //definitely, false is false
}
if(!undefined) {
    //undefined is false
}
if(!null) {
    //null is false
}
if(!'') {
    //empty string '' is false
}
if(!0) {
    //numver 0 is false
}
if(!NaN) {
    //numver NaN is false
}

Except these value, others could be regard as true.

ref: http://stackoverflow.com/questions/5101948/javascript-checking-for-null-vs-undefined-and-difference-between-and