CODEWITHSUNDEEP
javascriptconditional-statements
Co Worker
Co Worker

Reputation: 139

Javascript - What is the checking order for the condition statement

I'm so wonder about the condition statement in Javascript. I have two example as below:

var myObj = {};
console.log(myObj.pro != null && myObj.pro.val != null ? 0 : 1);
console.log(myObj.pro.val != null && myObj.pro != null ? 0 : 1);

The second line will print a value, but the third line will raise an exception. Is the condition checked from left to right? How is it checked when we have a lot of condition operators such as OR, NOT, (), AND in the same line. Which one will be checked first? Please correct me if I'm misunderstood.

Update: I add more complex example:

var !(!A || B && C && D) ? 0 : 1;

Upvotes: 2

Views: 398

Answers (8)

Komal Bansal
Komal Bansal

Reputation: 809

var myObj = {};
console.log(myObj.pro != null && (code which is not read)? 0 : 1);

In you first case, the code is executed something like above , myObj.pro !=null is false , thus without running anything else it gives 1

console.log(myObj.pro.val != null && myObj.pro != null ? 0 : 1);

In you second case when it tries to read pro.val (pro is undefined thus don't have any properties to access) gives error. When you try to access a property of undefined this error is returned.Example 

var obj;
console.log(obj) // undefined;
obj.key // Cannot read property 'key' of undefined(obj is undefined in this case)

Upvotes: 1

kator
kator

Reputation: 959

For the second line console.log(myObj.pro != null && myObj.pro.val != null ? 0 : 1);, myObj.pro evaluates to undefined which is then compared to null and that returns false. We know that in an AND comparison, Javascript ignores the right hand side of the AND when the left hand side evaluates to false. This reasonable behaviour prevents the interpreter from encountering the exception-bound right hand side code which is essentially undefined.val

The third line just goes ahead and evaluates undefined.val which throws an exception and halts interpretation.

If you are interested in knowing about precedence, take a look at this MSDN table

Upvotes: 3

Nikhil Kinkar
Nikhil Kinkar

Reputation: 781

What is happening here is like this,

console.log(myObj.pro != null && myObj.pro.val != null ? 0 : 1);

your && separates 2 conditions like this (myObj.pro != null)[consider 'A'] && (myObj.pro.val != null ? 0 : 1)[consider 'B'].

console.log(A && B ? 0 : 1);

so A gives null, && don't consider going to B, since it got false value from A.

console.log(B && A ? 0 : 1);

but here you are trying to check value of undefined (in this case myObj.pro is undefined) that throws the error.

hope this helps you.

Update

var !(!A || B && C && D) ? 0 : 1;

assuming that all A,B,C,D have true values, here we go from left to right step by step:

!A => false

false || B => true

true && C => true

true && D => true

!(true) => false

var false ? 0 : 1;

you get 0

Upvotes: 0

jsadev.net
jsadev.net

Reputation: 2590

var myObj = {};

Note: myObj.pro is undefined.

console.log(myObj.pro.val != null && myObj.pro != null ? 0 : 1);

thorws exeption beacause it can't check myObj.pro.val first because myObj.pro is undefined. (cant find .val of undefined)

its like Looking for undefined.val thats why you've got en exeption.

Upvotes: 0

Jay Suchak
Jay Suchak

Reputation: 172

try this

var myObj = {};

console.log(myObj.pro != null && myObj.pro.val != null ? 0 : 1);

console.log(myObj.pro != null && myObj.pro.val != null && myObj.pro != null ? 0 : 1);

Upvotes: 0

Santhosh mp
Santhosh mp

Reputation: 589

Its not about the order. In your example, myObj = {};

Hence it does not have property pro (That is myObj.pro is null) Which causes the below statements first condition myObj.pro != null to fail. Since the condition used is && the next condition is not tested and it returns 1.

In the next statement the first condition is myObj.pro.val != null. Since the object myObj does not have property pro and val which is being accessed, its throwing an exception.

Upvotes: 0

Naor Tedgi
Naor Tedgi

Reputation: 5707

https://en.wikipedia.org/wiki/AND_gate

A   B   A AND B
0   0   0
0   1   0
1   0   0
1   1   1

let's mark it like this 

A = myObj.pro != null 

B = myObj.pro.val != null

console.log(A && B != null ? 0 : 1);

because A is false we arent checking B here because false && something always be false

that's way you don't get an exception because B not even tested

in the second term 

console.log(B && A != null ? 0 : 1);

B is tested before A and thorws exception because myObj.pro is undefined and you traying to access myObj.pro.val

Upvotes: 3

Mohammad Albakri
Mohammad Albakri

Reputation: 79

I believe it will get executed from left to right. But you have an Undefined property val . It will start with myObj.pro which will be undefined. Then undefined.val will definitely raise the exception here. In your second line, that didn't raise an exception because it didn't pass the first Conditional statement

Upvotes: 0

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