Why does comparing value with undefined returns false in JavaScript?

I am new to Javascript and I noticed when a variable is undefined, comparing a number returns false as below. Why does comparing undefined with numbers return false?

var a = undefined;

console.log(a < 10);

console.log(10 < a);

console.log(a == 10);

edited Nov 20 '18 at 5:21

Towkir Ahmed

949620

asked Nov 20 '18 at 4:37

Poream3387

523214

2

What would you expect those to return? Certainly not true

– charlietfl
Nov 20 '18 at 4:41

Read: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

– Randy Casburn
Nov 20 '18 at 4:41

Because you are comparing the global undefined property that represents the primitive value undefined against a number and that returns false.. More about undefined

– Yosvel Quintero
Nov 20 '18 at 4:44

add a comment |

I am new to Javascript and I noticed when a variable is undefined, comparing a number returns false as below. Why does comparing undefined with numbers return false?

var a = undefined;

console.log(a < 10);

console.log(10 < a);

console.log(a == 10);

edited Nov 20 '18 at 5:21

Towkir Ahmed

949620

asked Nov 20 '18 at 4:37

Poream3387

523214

2

What would you expect those to return? Certainly not true

– charlietfl
Nov 20 '18 at 4:41

Read: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

– Randy Casburn
Nov 20 '18 at 4:41

Because you are comparing the global undefined property that represents the primitive value undefined against a number and that returns false.. More about undefined

– Yosvel Quintero
Nov 20 '18 at 4:44

add a comment |

I am new to Javascript and I noticed when a variable is undefined, comparing a number returns false as below. Why does comparing undefined with numbers return false?

var a = undefined;

console.log(a < 10);

console.log(10 < a);

console.log(a == 10);

edited Nov 20 '18 at 5:21

Towkir Ahmed

949620

asked Nov 20 '18 at 4:37

Poream3387

523214

I am new to Javascript and I noticed when a variable is undefined, comparing a number returns false as below. Why does comparing undefined with numbers return false?

var a = undefined;

console.log(a < 10);

console.log(10 < a);

console.log(a == 10);

var a = undefined;

console.log(a < 10);

console.log(10 < a);

console.log(a == 10);

var a = undefined;

console.log(a < 10);

console.log(10 < a);

console.log(a == 10);

javascript comparison undefined

edited Nov 20 '18 at 5:21

Towkir Ahmed

949620

asked Nov 20 '18 at 4:37

Poream3387

523214

edited Nov 20 '18 at 5:21

Towkir Ahmed

949620

asked Nov 20 '18 at 4:37

Poream3387

523214

edited Nov 20 '18 at 5:21

Towkir Ahmed

949620

edited Nov 20 '18 at 5:21

Towkir Ahmed

949620

edited Nov 20 '18 at 5:21

Towkir Ahmed

949620

asked Nov 20 '18 at 4:37

Poream3387

523214

asked Nov 20 '18 at 4:37

Poream3387

523214

asked Nov 20 '18 at 4:37

Poream3387

523214

2

What would you expect those to return? Certainly not true

– charlietfl
Nov 20 '18 at 4:41

Read: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

– Randy Casburn
Nov 20 '18 at 4:41

Because you are comparing the global undefined property that represents the primitive value undefined against a number and that returns false.. More about undefined

– Yosvel Quintero
Nov 20 '18 at 4:44

add a comment |

2

What would you expect those to return? Certainly not true

– charlietfl
Nov 20 '18 at 4:41

Read: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

– Randy Casburn
Nov 20 '18 at 4:41

Because you are comparing the global undefined property that represents the primitive value undefined against a number and that returns false.. More about undefined

– Yosvel Quintero
Nov 20 '18 at 4:44

What would you expect those to return? Certainly not true

– charlietfl
Nov 20 '18 at 4:41

Read: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

– Randy Casburn
Nov 20 '18 at 4:41

Because you are comparing the global undefined property that represents the primitive value undefined against a number and that returns false.. More about undefined

– Yosvel Quintero
Nov 20 '18 at 4:44

add a comment |

1 Answer
1

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oldest

votes

This is how works in JavaScript.

Number(undefined) // NaN

NaN == NaN // false

NaN < 0 // false

NaN > 0 // false

So, while you compare it forces to check like:

Number(undefined) < 10

// undefined is coerced to check with number

And thus,

undefined == 10 // false

undefined > 10 // false

undefined < 10 // false

answered Nov 20 '18 at 4:49

Bhojendra Rauniyar

50.8k2079125

add a comment |

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1 Answer
1

active

oldest

votes

1 Answer
1

active

oldest

votes

This is how works in JavaScript.

Number(undefined) // NaN

NaN == NaN // false

NaN < 0 // false

NaN > 0 // false

So, while you compare it forces to check like:

Number(undefined) < 10

// undefined is coerced to check with number

And thus,

undefined == 10 // false

undefined > 10 // false

undefined < 10 // false

answered Nov 20 '18 at 4:49

Bhojendra Rauniyar

50.8k2079125

add a comment |

This is how works in JavaScript.

Number(undefined) // NaN

NaN == NaN // false

NaN < 0 // false

NaN > 0 // false

So, while you compare it forces to check like:

Number(undefined) < 10

// undefined is coerced to check with number

And thus,

undefined == 10 // false

undefined > 10 // false

undefined < 10 // false

answered Nov 20 '18 at 4:49

Bhojendra Rauniyar

50.8k2079125

add a comment |

This is how works in JavaScript.

Number(undefined) // NaN

NaN == NaN // false

NaN < 0 // false

NaN > 0 // false

So, while you compare it forces to check like:

Number(undefined) < 10

// undefined is coerced to check with number

And thus,

undefined == 10 // false

undefined > 10 // false

undefined < 10 // false

answered Nov 20 '18 at 4:49

Bhojendra Rauniyar

50.8k2079125

This is how works in JavaScript.

Number(undefined) // NaN

NaN == NaN // false

NaN < 0 // false

NaN > 0 // false

So, while you compare it forces to check like:

Number(undefined) < 10

// undefined is coerced to check with number

And thus,

undefined == 10 // false

undefined > 10 // false

undefined < 10 // false

answered Nov 20 '18 at 4:49

Bhojendra Rauniyar

50.8k2079125

answered Nov 20 '18 at 4:49

Bhojendra Rauniyar

50.8k2079125

answered Nov 20 '18 at 4:49

Bhojendra Rauniyar

50.8k2079125

answered Nov 20 '18 at 4:49

Bhojendra Rauniyar

50.8k2079125

add a comment |

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