In if statement,undefined equals with false

I'm confused with the code below:

if(undefined){

   //code will not be executed

}

and

if(!undefined){

   //code will be executed

}

Is that mean the "undefined" equals with false?

Here the question related,but no one point above situation out.

edited May 23 '17 at 10:31

Community♦

asked May 11 '16 at 15:40

Xheldon Cao

779

3

The truthy value of undefined in Javascript is false. Hence the behaviour that you see.
– stackErr
May 11 '16 at 15:42

1

Boolean(undefined) is false.but your comments should be interchanged
– Madhawa Priyashantha
May 11 '16 at 15:43

@FastSnail Is that means if statement will switch every parameter passed to it inside?I mean,just like this:` if(Boolean(parameter)){//code}`,right?
– Xheldon Cao
May 12 '16 at 2:25

@XheldonCao if you pass a none Boolean value then yes.javascript use Boolean() function to get a boolean value because if condition need a boolean value
– Madhawa Priyashantha
May 12 '16 at 3:18

@FastSnail Now I know,thanks guys!
– Xheldon Cao
May 12 '16 at 3:44

add a comment |

I'm confused with the code below:

if(undefined){

   //code will not be executed

}

and

if(!undefined){

   //code will be executed

}

Is that mean the "undefined" equals with false?

Here the question related,but no one point above situation out.

edited May 23 '17 at 10:31

Community♦

asked May 11 '16 at 15:40

Xheldon Cao

779

3

The truthy value of undefined in Javascript is false. Hence the behaviour that you see.
– stackErr
May 11 '16 at 15:42

1

Boolean(undefined) is false.but your comments should be interchanged
– Madhawa Priyashantha
May 11 '16 at 15:43

@FastSnail Is that means if statement will switch every parameter passed to it inside?I mean,just like this:` if(Boolean(parameter)){//code}`,right?
– Xheldon Cao
May 12 '16 at 2:25

@XheldonCao if you pass a none Boolean value then yes.javascript use Boolean() function to get a boolean value because if condition need a boolean value
– Madhawa Priyashantha
May 12 '16 at 3:18

@FastSnail Now I know,thanks guys!
– Xheldon Cao
May 12 '16 at 3:44

add a comment |

I'm confused with the code below:

if(undefined){

   //code will not be executed

}

and

if(!undefined){

   //code will be executed

}

Is that mean the "undefined" equals with false?

Here the question related,but no one point above situation out.

edited May 23 '17 at 10:31

Community♦

asked May 11 '16 at 15:40

Xheldon Cao

779

I'm confused with the code below:

if(undefined){

   //code will not be executed

}

and

if(!undefined){

   //code will be executed

}

Is that mean the "undefined" equals with false?

Here the question related,but no one point above situation out.

javascript if-statement undefined

edited May 23 '17 at 10:31

Community♦

asked May 11 '16 at 15:40

Xheldon Cao

779

edited May 23 '17 at 10:31

Community♦

asked May 11 '16 at 15:40

Xheldon Cao

779

edited May 23 '17 at 10:31

Community♦

edited May 23 '17 at 10:31

Community♦

edited May 23 '17 at 10:31

Community♦

asked May 11 '16 at 15:40

Xheldon Cao

779

asked May 11 '16 at 15:40

Xheldon Cao

779

asked May 11 '16 at 15:40

Xheldon Cao

779

3

The truthy value of undefined in Javascript is false. Hence the behaviour that you see.
– stackErr
May 11 '16 at 15:42

1

Boolean(undefined) is false.but your comments should be interchanged
– Madhawa Priyashantha
May 11 '16 at 15:43

@FastSnail Is that means if statement will switch every parameter passed to it inside?I mean,just like this:` if(Boolean(parameter)){//code}`,right?
– Xheldon Cao
May 12 '16 at 2:25

@XheldonCao if you pass a none Boolean value then yes.javascript use Boolean() function to get a boolean value because if condition need a boolean value
– Madhawa Priyashantha
May 12 '16 at 3:18

@FastSnail Now I know,thanks guys!
– Xheldon Cao
May 12 '16 at 3:44

add a comment |

3

The truthy value of undefined in Javascript is false. Hence the behaviour that you see.
– stackErr
May 11 '16 at 15:42

1

Boolean(undefined) is false.but your comments should be interchanged
– Madhawa Priyashantha
May 11 '16 at 15:43

@FastSnail Is that means if statement will switch every parameter passed to it inside?I mean,just like this:` if(Boolean(parameter)){//code}`,right?
– Xheldon Cao
May 12 '16 at 2:25

@XheldonCao if you pass a none Boolean value then yes.javascript use Boolean() function to get a boolean value because if condition need a boolean value
– Madhawa Priyashantha
May 12 '16 at 3:18

@FastSnail Now I know,thanks guys!
– Xheldon Cao
May 12 '16 at 3:44

The truthy value of undefined in Javascript is false. Hence the behaviour that you see.
– stackErr
May 11 '16 at 15:42

Boolean(undefined) is false.but your comments should be interchanged
– Madhawa Priyashantha
May 11 '16 at 15:43

@FastSnail Is that means if statement will switch every parameter passed to it inside?I mean,just like this:` if(Boolean(parameter)){//code}`,right?
– Xheldon Cao
May 12 '16 at 2:25

@XheldonCao if you pass a none Boolean value then yes.javascript use Boolean() function to get a boolean value because if condition need a boolean value
– Madhawa Priyashantha
May 12 '16 at 3:18

@FastSnail Now I know,thanks guys!
– Xheldon Cao
May 12 '16 at 3:44

add a comment |

3 Answers
3

active

oldest

votes

It means that undefined is a falsy value, list of falsy values are:

""        # Empty string

null      # null

undefined # undefined, which you get when doing: var a;

false     # Boolean false

0         # Number 0

NaN       # Not A Number eg: "a" * 2

If you negate a falsy value you will get true:

!""        === true

!null      === true

!undefined === true

!0         === true

!NaN       === true

And when you nagate a truly value you will get false:

!"hello" === false

!1       === false

But undefined is not equal false:

undefined === false // false

undefined  == false // false

And just for the fun if it:

undefined == null // true

edited May 11 '16 at 16:05

answered May 11 '16 at 15:43

andlrc

35.5k126498

I will write it down my memorandum，thanks
– Xheldon Cao
May 12 '16 at 2:46

add a comment |

In javascript strict mode, undefined is not false, but javascript try to convert the object or var to a boolean value (this is called in javascript truthy value), that's the reason you got an undefined as false. This happens with null also, for example.

You can force that with this strict no equality:

if(undefined!==false) console.log("Is not false");

answered May 11 '16 at 15:47

Christopher Díaz

31817

add a comment |

Please take a look below checked falsy values:

""==false?

    Ans: true

null == false?

    Ans: false

undefined == false?

    Ans: false

0 == false?

    Ans: true

NaN == false?

    Ans: false

null == NaN?

    Ans: false

We can see that null == false,undefined == false,null == NaN, and NaN == false are not true
That means they are not equal. From the above result, we got 3 falsy values group:

The False group

The Null group and

The NaN group

But a negative falsy value is always true:

!""        === true

!null      === true

!undefined === true

!0         === true

!NaN       === true

For example:
To check true value of dataTitle variable

if(dataTitle && (dataTitle != null))

{

    console.log('hi');

}

The above statement will check the false group as well as the null group

To check false value of dataTitle variable

if(!dataTitle)

{

    console.log('hi');

}

edited Nov 20 at 10:11

answered Nov 20 at 9:04

Bablu Ahmed

940720

add a comment |

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3 Answers
3

active

oldest

votes

3 Answers
3

active

oldest

votes

It means that undefined is a falsy value, list of falsy values are:

""        # Empty string

null      # null

undefined # undefined, which you get when doing: var a;

false     # Boolean false

0         # Number 0

NaN       # Not A Number eg: "a" * 2

If you negate a falsy value you will get true:

!""        === true

!null      === true

!undefined === true

!0         === true

!NaN       === true

And when you nagate a truly value you will get false:

!"hello" === false

!1       === false

But undefined is not equal false:

undefined === false // false

undefined  == false // false

And just for the fun if it:

undefined == null // true

edited May 11 '16 at 16:05

answered May 11 '16 at 15:43

andlrc

35.5k126498

I will write it down my memorandum，thanks
– Xheldon Cao
May 12 '16 at 2:46

add a comment |

It means that undefined is a falsy value, list of falsy values are:

""        # Empty string

null      # null

undefined # undefined, which you get when doing: var a;

false     # Boolean false

0         # Number 0

NaN       # Not A Number eg: "a" * 2

If you negate a falsy value you will get true:

!""        === true

!null      === true

!undefined === true

!0         === true

!NaN       === true

And when you nagate a truly value you will get false:

!"hello" === false

!1       === false

But undefined is not equal false:

undefined === false // false

undefined  == false // false

And just for the fun if it:

undefined == null // true

edited May 11 '16 at 16:05

answered May 11 '16 at 15:43

andlrc

35.5k126498

I will write it down my memorandum，thanks
– Xheldon Cao
May 12 '16 at 2:46

add a comment |

It means that undefined is a falsy value, list of falsy values are:

""        # Empty string

null      # null

undefined # undefined, which you get when doing: var a;

false     # Boolean false

0         # Number 0

NaN       # Not A Number eg: "a" * 2

If you negate a falsy value you will get true:

!""        === true

!null      === true

!undefined === true

!0         === true

!NaN       === true

And when you nagate a truly value you will get false:

!"hello" === false

!1       === false

But undefined is not equal false:

undefined === false // false

undefined  == false // false

And just for the fun if it:

undefined == null // true

edited May 11 '16 at 16:05

answered May 11 '16 at 15:43

andlrc

35.5k126498

It means that undefined is a falsy value, list of falsy values are:

""        # Empty string

null      # null

undefined # undefined, which you get when doing: var a;

false     # Boolean false

0         # Number 0

NaN       # Not A Number eg: "a" * 2

If you negate a falsy value you will get true:

!""        === true

!null      === true

!undefined === true

!0         === true

!NaN       === true

And when you nagate a truly value you will get false:

!"hello" === false

!1       === false

But undefined is not equal false:

undefined === false // false

undefined  == false // false

And just for the fun if it:

undefined == null // true

edited May 11 '16 at 16:05

answered May 11 '16 at 15:43

andlrc

35.5k126498

edited May 11 '16 at 16:05

answered May 11 '16 at 15:43

andlrc

35.5k126498

answered May 11 '16 at 15:43

andlrc

35.5k126498

answered May 11 '16 at 15:43

andlrc

35.5k126498

I will write it down my memorandum，thanks
– Xheldon Cao
May 12 '16 at 2:46

add a comment |

I will write it down my memorandum，thanks
– Xheldon Cao
May 12 '16 at 2:46

I will write it down my memorandum，thanks
– Xheldon Cao
May 12 '16 at 2:46

add a comment |

You can force that with this strict no equality:

if(undefined!==false) console.log("Is not false");

answered May 11 '16 at 15:47

Christopher Díaz

31817

add a comment |

You can force that with this strict no equality:

if(undefined!==false) console.log("Is not false");

answered May 11 '16 at 15:47

Christopher Díaz

31817

add a comment |

You can force that with this strict no equality:

if(undefined!==false) console.log("Is not false");

answered May 11 '16 at 15:47

Christopher Díaz

31817

You can force that with this strict no equality:

if(undefined!==false) console.log("Is not false");

answered May 11 '16 at 15:47

Christopher Díaz

31817

answered May 11 '16 at 15:47

Christopher Díaz

31817

answered May 11 '16 at 15:47

Christopher Díaz

31817

answered May 11 '16 at 15:47

Christopher Díaz

31817

add a comment |

Please take a look below checked falsy values:

""==false?

    Ans: true

null == false?

    Ans: false

undefined == false?

    Ans: false

0 == false?

    Ans: true

NaN == false?

    Ans: false

null == NaN?

    Ans: false

We can see that null == false,undefined == false,null == NaN, and NaN == false are not true
That means they are not equal. From the above result, we got 3 falsy values group:

The False group

The Null group and

The NaN group

But a negative falsy value is always true:

!""        === true

!null      === true

!undefined === true

!0         === true

!NaN       === true

For example:
To check true value of dataTitle variable

if(dataTitle && (dataTitle != null))

{

    console.log('hi');

}

The above statement will check the false group as well as the null group

To check false value of dataTitle variable

if(!dataTitle)

{

    console.log('hi');

}

edited Nov 20 at 10:11

answered Nov 20 at 9:04

Bablu Ahmed

940720

add a comment |

Please take a look below checked falsy values:

""==false?

    Ans: true

null == false?

    Ans: false

undefined == false?

    Ans: false

0 == false?

    Ans: true

NaN == false?

    Ans: false

null == NaN?

    Ans: false

We can see that null == false,undefined == false,null == NaN, and NaN == false are not true
That means they are not equal. From the above result, we got 3 falsy values group:

The False group

The Null group and

The NaN group

But a negative falsy value is always true:

!""        === true

!null      === true

!undefined === true

!0         === true

!NaN       === true

For example:
To check true value of dataTitle variable

if(dataTitle && (dataTitle != null))

{

    console.log('hi');

}

The above statement will check the false group as well as the null group

To check false value of dataTitle variable

if(!dataTitle)

{

    console.log('hi');

}

edited Nov 20 at 10:11

answered Nov 20 at 9:04

Bablu Ahmed

940720

add a comment |

Please take a look below checked falsy values:

""==false?

    Ans: true

null == false?

    Ans: false

undefined == false?

    Ans: false

0 == false?

    Ans: true

NaN == false?

    Ans: false

null == NaN?

    Ans: false

We can see that null == false,undefined == false,null == NaN, and NaN == false are not true
That means they are not equal. From the above result, we got 3 falsy values group:

The False group

The Null group and

The NaN group

But a negative falsy value is always true:

!""        === true

!null      === true

!undefined === true

!0         === true

!NaN       === true

For example:
To check true value of dataTitle variable

if(dataTitle && (dataTitle != null))

{

    console.log('hi');

}

The above statement will check the false group as well as the null group

To check false value of dataTitle variable

if(!dataTitle)

{

    console.log('hi');

}

edited Nov 20 at 10:11

answered Nov 20 at 9:04

Bablu Ahmed

940720

Please take a look below checked falsy values:

""==false?

    Ans: true

null == false?

    Ans: false

undefined == false?

    Ans: false

0 == false?

    Ans: true

NaN == false?

    Ans: false

null == NaN?

    Ans: false

We can see that null == false,undefined == false,null == NaN, and NaN == false are not true
That means they are not equal. From the above result, we got 3 falsy values group:

The False group

The Null group and

The NaN group

But a negative falsy value is always true:

!""        === true

!null      === true

!undefined === true

!0         === true

!NaN       === true

For example:
To check true value of dataTitle variable

if(dataTitle && (dataTitle != null))

{

    console.log('hi');

}

The above statement will check the false group as well as the null group

To check false value of dataTitle variable

if(!dataTitle)

{

    console.log('hi');

}

edited Nov 20 at 10:11

answered Nov 20 at 9:04

Bablu Ahmed

940720

edited Nov 20 at 10:11

answered Nov 20 at 9:04

Bablu Ahmed

940720

answered Nov 20 at 9:04

Bablu Ahmed

940720

answered Nov 20 at 9:04

Bablu Ahmed

940720

add a comment |

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